A piece of paper inside of a ziploc bag should work. I think my spanish teacher did that one time.
My son used a piece of saran wrap over a piece of white cardboard.
I was going to suggest what Burf said, but I'll go with a different method (grumble grumble grumble) Break into a school.
A piece of glass scavenged from an old window or picture frame secured on top of a sheet of white paper makes an excellent white board.
An offcut of white melamine faced particle board works.