3/4in is what I used on mine. A lot of people double it up or brace with 2x4. If you fiberglass though you could probably get away with a single layer.
3/4" is what I am using. Home Depot had sanded cabinet grade plywood for $30 last time I was in there. You can use 2x4 or 2x6 for the stand, I went with 2x4's. You should start a plywood thread on the diy subforum, a lot of builders hang out there that may not catch your thread here.