I used to have a Subaru wagon and ended up buying one of those plastic carpet savers for office chairs, and cutting it (don't ask, what a pita) to fit so that I could slide my bike in without it messing up the carpet. It worked well and was easy to wipe clean too. I think some companies make pre-molded plastic carpet savers for cars but they were more than I wanted to spend at the time. I would then cover it with a tarp to keep it hidden.