Kevin,
I think you were under-inflated for those KMs and that heavy a truck.
I had my Michelin LTX MS2s (E rated) aired down to 40 psi and cannot recall any spot on that trail where I was lacking traction. (although with occasional help from the rear locker when crossed up)
may work as-is, 1/8" is pretty thin ply, or apply strips cut across the short length of the panel to keep the exterior grain perpendiclar to the curve.
I must admit, though, the fiberglass and resin came to mind as well. Maybe both?
if it does not have to support any pressure then maybe use a door skin, 1/8" lauan plywood designed to cover doors with a pretty veneer.
http://www.lowes.com/pd_80037-77999-80037_0__?productId=3556804
I spend so much time wandering the isles of HD/Lowes looking for things to use in ways they...