Masonite is an old standby, cheap, ridgid and fairly easy to cut. Lauan plywood is also an easy product to work with. You can buy it at any home center in 1/8" 4' x 8' sheets, or smaller "door skins" for just a bit more per square foot than masonite. I like it because it takes paint a lot better than masonite, and it doesn't have the rough backing that masonite does.
I had my layout wraped in masonite, and found that I put it on way too soon.
Kept taking it off little by little as it got in the way. Now there is none left.
I will use lauan as soon as I think I can, and not a month sooner....