Seat advice
I would reccomend using 3/4" exterior grade or marine grade plywood. Install stainless steel tee nuts before you cover it, and paint the bottom side black to help seal out moisture. Also, using Stainless Steel or Monel staples is a nice finishing touch, they will never rust out.
Also, I may be Captain Overkill, but I also would round all corners to a radius of 1/4" with a router or sandpaper to minimize stress on the vinyl.
This is how I made mine and it looks great on the bike.