Is this to be aesthetically pleasing or just functional?
Could be as simple as a couple pieces of plywood. You could put some neoprene strips (or anything similar)on the bottom side... or even go crazy and cover it in carpet. You could also epoxy the wood (and, if you wanted, use some glass) - and then paint and add non-skid for the top side.
PVC board is an option. HDPE board is an option. With PVC, you can strengthen unsupported areas by gluing another piece/strip of board to the area or even using L-channel. For HDPE you can't glue - so use thru-bolts. I'd still use some thru-bolts with the PVC, as well, though.