Uploaded November 2025 | Updated September 2026, 2 weeks ago
Learn how to make a professional-looking box cushion using marine-grade flotation foam. Perfect for boats, docks, or poolside seating! Follow along step-by-step as we cut, sew, and assemble a durable, water-resistant cushion that’s built to float and last.
The guidelines for CushionRite Flotation Foam / Closed Cell Foam are slightly different than a soft to firm foam's guidelines due to its exceptional firmness. This can also be applied when utilizing an extra-firm regular upholstery foam. Below are the guidelines for box cushions when using an extra-firm foam:
1. Cut foam ¼” larger than the desired finished size on all sides. The cover will compress the foam slightly, bringing it very close to your desired finished size. For example: If my desired finished size is 22” x 24”, I will cut my foam to 22 ½” x 24 ½”.
2. Cut (top and bottom) fabric plates ¼” larger on all sides than the oversized foam size.
3. Cut boxing width ¾”, thicker than the foam’s thickness. If you add a polyester batting to the top and/or top/bottom, include half the thickness of the combined batting thickness to the boxing width mentioned.
4. The length of the boxing should equal all three sides of the cushion plus 4 inches, not including the side for the zipper.
5. Cut the zipper plaque boxing to the same width as my boxing width above, plus the width of the zipper used. (example 3" closed cell foam + 3/4" + zipper width = cut width)
6. The length of the zipper plaque boxing should be the length of the side it will be installed on, plus approximately 10 inches. It will wrap around to the sides by a few inches.
Products:
Sattler Fabrics - sailrite.com/shop/fabric/marine-fabric?facet_brand=Sattler?utm_source=DIY&utm_medium=Float_BOXCU
Blade Foam Saw - sailrite.com/Sailrite-Blade-8-Foam-Cutter-with-Carrying-Case-110V?utm_source=DIY&utm_medium=Float_BOXCU
CushionRite Flotation Foam - sailrite.com/CushionRite-Flotation-Antimicrobial-Foam-Extra-Firm?utm_source=DIY&utm_medium=Float_BOXCU
CushionRite Spray - sailrite.com/CushionRite-Foam-Fabric-Spray-Adhesive?utm_source=DIY&utm_medium=Float_BOXCU
Foam Batting - sailrite.com/Batting-83-Thick-55-Wide?utm_source=DIY&utm_medium=Float_BOXCU
Measuring Tape - sailrite.com/Tape-Measure-120-Inch?utm_source=DIY&utm_medium=Float_BOXCU
Zipper Chain - sailrite.com/Lenzip-5-Royal-Blue-Continuous-Zipper-Chain-Molded-Tooth?utm_source=DIY&utm_medium=Float_BOXCU
Precision Cutting Foot - sailrite.com/Precision-Cutting-Blade-Foot-for-Hotknives?utm_source=DIY&utm_medium=Float_BOXCU
Hotknife - sailrite.com/Sailrite-Edge-Cordless-Hotknife-18V-Package?utm_source=DIY&utm_medium=Float_BOXCU
Fabric Notching Tool - sailrite.com/Fabric-Edge-Pattern-Notching-Tool?utm_source=DIY&utm_medium=Float_BOXCU
Sewing Machine - sailrite.com/Sailrite-Ultrafeed-LSZ-Walking-Foot-Sewing-Machine-110V?utm_source=DIY&utm_medium=Float_BOXCU
Clear Acrylic Ruler - sailrite.com/Clear-Acrylic-Ruler-6-x-24?utm_source=DIY&utm_medium=Float_BOXCU
Chalk Pen - sailrite.com/Magic-Chalk-Liner-White?utm_source=DIY&utm_medium=Float_BOXCU
Basting Tape - sailrite.com/Seamstick-1-4-Basting-Tape-for-Canvas-50-Yds?utm_source=DIY&utm_medium=Float_BOXCU
Canvas Patterning Ruler - sailrite.com/Sailrite-Canvas-Patterning-Ruler?utm_source=DIY&utm_medium=Float_BOXCU
Thread Snips - sailrite.com/Basic-Thread-Snips?utm_source=DIY&utm_medium=Float_BOXCU
Locking Zipper Pull - sailrite.com/Lenzip-5-Royal-Blue-Single-Locking-Metal-Zipper-Pull-Molded-Tooth-Chain_2?utm_source=DIY&utm_medium=Float_BOXCU
Eversoft Piping - sailrite.com/EverSoft-Smooth-Vinyl-Welting-Piping-White?utm_source=DIY&utm_medium=Float_BOXCU
Sewing Gauge Ruler - sailrite.com/Sewing-Gauge-Ruler-6?utm_source=DIY&utm_medium=Float_BOXCU
Magnetic Sewing Guide - sailrite.com/Deluxe-5-1-2-Magnetic-Sewing-Guide?utm_source=DIY&utm_medium=Float_BOXCU
Fabric Edge Match Pliers - sailrite.com/Fabric-Edge-Match-Pliers?utm_source=DIY&utm_medium=Float_BOXCU
Learn how to make a professional-looking box cushion using marine-grade flotation foam. Perfect for boats, docks, or poolside seating! Follow along step-by-step as we cut, sew, and assemble a durable, water-resistant cushion that’s built to float and last.
The guidelines for CushionRite Flotation Foam / Closed Cell Foam are slightly different than a soft to firm foam's guidelines due to its exceptional firmness. This can also be applied when utilizing an extra-firm regular upholstery foam. Below are the guidelines for box cushions when using an extra-firm foam:
1. Cut foam ¼” larger than the desired finished size on all sides. The cover will compress the foam slightly, bringing it very close to your desired finished size. For example: If my desired finished size is 22” x 24”, I will cut my foam to 22 ½” x 24 ½”.
2. Cut (top and bottom) fabric plates ¼” larger on all sides than the oversized foam size.
3. Cut boxing width ¾”, thicker than the foam’s thickness. If you add a polyester batting to the top and/or top/bottom, include half the thickness of the combined batting thickness to the boxing width mentioned.
4. The length of the boxing should equal all three sides of the cushion plus 4 inches, not including the side for the zipper.
5. Cut the zipper plaque boxing to the same width as my boxing width above, plus the width of the zipper used. (example 3" closed cell foam + 3/4" + zipper width = cut width)
6. The length of the zipper plaque boxing should be the length of the side it will be installed on, plus approximately 10 inches. It will wrap around to the sides by a few inches.
Products:
Sattler Fabrics - sailrite.com/shop/fabric/marine-fabric?facet_brand=Sattler?utm_source=DIY&utm_medium=Float_BOXCU
Blade Foam Saw - sailrite.com/Sailrite-Blade-8-Foam-Cutter-with-Carrying-Case-110V?utm_source=DIY&utm_medium=Float_BOXCU
CushionRite Flotation Foam - sailrite.com/CushionRite-Flotation-Antimicrobial-Foam-Extra-Firm?utm_source=DIY&utm_medium=Float_BOXCU
CushionRite Spray - sailrite.com/CushionRite-Foam-Fabric-Spray-Adhesive?utm_source=DIY&utm_medium=Float_BOXCU
Foam Batting - sailrite.com/Batting-83-Thick-55-Wide?utm_source=DIY&utm_medium=Float_BOXCU
Measuring Tape - sailrite.com/Tape-Measure-120-Inch?utm_source=DIY&utm_medium=Float_BOXCU
Zipper Chain - sailrite.com/Lenzip-5-Royal-Blue-Continuous-Zipper-Chain-Molded-Tooth?utm_source=DIY&utm_medium=Float_BOXCU
Precision Cutting Foot - sailrite.com/Precision-Cutting-Blade-Foot-for-Hotknives?utm_source=DIY&utm_medium=Float_BOXCU
Hotknife - sailrite.com/Sailrite-Edge-Cordless-Hotknife-18V-Package?utm_source=DIY&utm_medium=Float_BOXCU
Fabric Notching Tool - sailrite.com/Fabric-Edge-Pattern-Notching-Tool?utm_source=DIY&utm_medium=Float_BOXCU
Sewing Machine - sailrite.com/Sailrite-Ultrafeed-LSZ-Walking-Foot-Sewing-Machine-110V?utm_source=DIY&utm_medium=Float_BOXCU
Clear Acrylic Ruler - sailrite.com/Clear-Acrylic-Ruler-6-x-24?utm_source=DIY&utm_medium=Float_BOXCU
Chalk Pen - sailrite.com/Magic-Chalk-Liner-White?utm_source=DIY&utm_medium=Float_BOXCU
Basting Tape - sailrite.com/Seamstick-1-4-Basting-Tape-for-Canvas-50-Yds?utm_source=DIY&utm_medium=Float_BOXCU
Canvas Patterning Ruler - sailrite.com/Sailrite-Canvas-Patterning-Ruler?utm_source=DIY&utm_medium=Float_BOXCU
Thread Snips - sailrite.com/Basic-Thread-Snips?utm_source=DIY&utm_medium=Float_BOXCU
Locking Zipper Pull - sailrite.com/Lenzip-5-Royal-Blue-Single-Locking-Metal-Zipper-Pull-Molded-Tooth-Chain_2?utm_source=DIY&utm_medium=Float_BOXCU
Eversoft Piping - sailrite.com/EverSoft-Smooth-Vinyl-Welting-Piping-White?utm_source=DIY&utm_medium=Float_BOXCU
Sewing Gauge Ruler - sailrite.com/Sewing-Gauge-Ruler-6?utm_source=DIY&utm_medium=Float_BOXCU
Magnetic Sewing Guide - sailrite.com/Deluxe-5-1-2-Magnetic-Sewing-Guide?utm_source=DIY&utm_medium=Float_BOXCU
Fabric Edge Match Pliers - sailrite.com/Fabric-Edge-Match-Pliers?utm_source=DIY&utm_medium=Float_BOXCU
