"The brown seat pad was home made. The covering is vinyl from Joanne's Fabric and my wife did the stitch pattern with her sewing machine. The tail light was bought on e-bay. Attached are some pics of the seat process."
The base of the pad is made from a marine material called Starboard. It's easily cut and can be formed with a heat gun, and nails/staples hold well in it.
The first layer of foam is actually from a few cheap gardening knee pads.
Then I put another layer of soft smoothing foam on that.
The foam is glued to the board with contact cement...
...and wrapped the vinyl around it (stapled underneath).
The vinyl is glued to the foam where it curves upward in the back to keep it flat. The vinyl was cut into strips and then sewn back together in the pattern you see here (stitching idea taken from THIS technique).