That is similar to what I did in my 72 Scamp. I cut the original foam cushions to fit my needs, bought nice bed sheets at my local thrift store, cut the plywood, and wrapped them up like Christmas presents. I removed the fiberglass
benches and replaced them with wood frames. I used 3/4" ply, since they needed to be supportive. I stapled the fabric to the undersides, and then used strips of plywood to Really hold the fabric and hold the removable cushions in place within their open frames. I hope that makes sense. If I were putting these down on fiberglass, I would use 3/8 or 1/2" ply for better staple hold, and tape over the staples to prevent scratching. Using this method, be sure to round over the plywood edges or set it back far enough to prevent the fabric from tearing on the edge.
All in all, a very inexpensive and easy way to make DIY cushions. Although not as nice as professionally made ones with tough fabric, I won't mind doing mine over again for 2 dollars spent at the thrift store and a little bit of labor.