I find that sport court tiles work well too. The come in 12"x12" size, so you can lock together as many as you need. I also use them as the floor of my shower shelter in camp. When not in use they can be interlocked in the correct size to fit the floor of the roofrack. Cheap, strong, flexible, multi-use.
I think I paid about $1.50 each for mine, which are somthing like these:
http://www.floorscore.com/outdoorcourt.html
Haven't used them in snow yet, but they work great in sand.