Here is a low cost, long lasting, quick, and easy no-sew way to make cockpit cushions. I made a set for my VN23 Chiquita about 20 years ago and they still look like new. I made the ones shown here for Monita about 4 years ago. They are made from three layers of closed cell foam camping pads, about $10 each at Walmart.
I made a cardboard pattern to get the curves right. I glued the three layers together using contact cement, then glued the Sunbrella fabric cover on with the same contact cement. The glue does not seep through the fabric. No need to cover the bottom. And there are no wrinkles whatsoever, with no sewing at all.