We made the ATV seat out of 3/4" plywood, a 2" layer of high density foam, and a 1" layer of softer foam.  We bought some black vinly at a fabric store:
Because of the bend , we cut the wood in two pieces, clamped in place, and then screwed together with wood screws - we reinforced the joint with epoxy:
We used T-nuts mounted on holes drilled in the plywood, and secured the seat with 1/4" bolts - the T-nuts must be placed prior to doing the seat cover:
Cut the foam, cut the fabric, and staple, staple, staple (with a staplegun):