HDPE is probably the best general use plastic, imo. I don't love it, but it will work. I'd go w/the black textured stuff, commonly branded as King Starboard. Minimum 3/8" thick I would guess.
If you find a magic plastic that is lighter and as strong as good plywood I'd love to hear about it, but in my hundreds of hours (literally) of research I haven't found it.
Another option that I did is a 1/8" layer of black abs sheet with a thick rubber truck bed mat glued to it. If I did it again I'd use nutserts and machine screws or go with Richlite. The mat is too heavy for the stock clips and pops loose sometimes. But it is a durable surface and it's easy on the knees.
If you find a magic plastic that is lighter and as strong as good plywood I'd love to hear about it, but in my hundreds of hours (literally) of research I haven't found it.
Another option that I did is a 1/8" layer of black abs sheet with a thick rubber truck bed mat glued to it. If I did it again I'd use nutserts and machine screws or go with Richlite. The mat is too heavy for the stock clips and pops loose sometimes. But it is a durable surface and it's easy on the knees.