if you look at the into parts to roam/ the collective esp darren berrecloth's parts it looks like there is chicken wire on the topside surface. i imagine it works quite well considering the size of the holes and the size of the tread of tires. seems like just enough to ensure you are gonna get grip if its wet.
i found a big board with 2.4 sides thats about 8x5 or so at the place i work. it is covered in a cheap rough carpet. i'm gonna use that for the landing of a jump that i'm building this week. i can't see that getting slick in the rain at all, so if you don't need to cover a ton of surface that might be a possiblility. chicken wire also should do the trick.