The cardboard stock I use is perfect for prototyping parts. It's very stiff and nearly the same thickness as 2mm Stratocore, and it's cheap (well, free with purchase of Bisquick from Costco).
Unfortunately there are also many puddles of standing water around the edges of our streets. My car seems to end up plowing through these puddles every time I make new parts to try out. It seems to be an inevitable outcome given the combination of speed, daring, nice drifty patches of dirt, and apparently poor water management.
Many prototype parts have died this way. Seeing the work dissolving into smelly pulp is a strong motivator for me to finish the design and fabricate the proper pieces from completely water-proof plastic.