I used cardboard and newspaper for a little bit, but that was a pain in the ass to put up/take down. I ended up making some curtains and stringing them up with some old clothes line. That way they're always there, but if you need to move your carhouse you can just slide them aside.
I guess it depends on the size of your vehicle, but it really doesn't take much fabric. You can usually find solid prints for as cheap as 2-3 bucks, then you can just hand sew it if you don't have access to a sewing machine.
I got lucky and found some insanely inexpensive pink fabric covered in odd looking bunny rabbits and melting carrots, that was apparently leftover from Easter time. Still confused as to how that never sold out...