This is a new seat cushion for my Step-Dad's tractor. Because the cushion was torn and rotting away, I had to start by buying a new piece of foam. Jo Ann Fabric had 5 inch thick foam that worked great. Here's a tip I learned: use an electric knife to cut the foam to the correct dimensions. I used a tan upholstery fabric to sew the cover (I can't remember exactly what it's called). Below is a picture of the backrest with 2 inch foam.
Because I didn't have the plywood backing, I left 3 inches of fabric that can be pulled tight and stapled with a staple gun to the plywood board.