There is a product out called "NuFoam" that would be perfect for this use. Depending on the size that you need, you can buy precut cushions of varying thicknesses, or you can buy the material by the yard and cut your own sizes. I know that WalMart had the precut squares in about a 1" thickness. The other products I saw at Hancock Fabrics. This product is mildew resistant, waterproof and perfect for outside cushions. I used 2" thick cushions when I reupholstered the seats on my dining room chairs. The old foam had shredded into non-existance. This NuFoam will not shred because it's like strings instead of rubber. You can probably google Nufoam and see what it's like.
Good luck with your project!