I rebuilt mine out of birch plywood soaked in Thompson's Water Seal, and covered them with carpet. See thread:
UHMW (HDPE) is not a good choice because it will creep under load. King Starboard is stiffer, but expensive and still not as stiff as plywood. Don't know if you're planning to carpet or not, but adhesives don't like plastic. Another choice would be honeycomb aluminum board, but it is very expensive and has to be thru-bolted.