I've been looking around online for ideas for making puppet theaters for my kids at work. I ran across this page by California Mandarins with some neat ideas for reusing the boxes as children's toys & crafts. If you're not into logos, you can always cover the boxes with paper or paint. Of course, you can use just about any kind of box to make these projects too, like for this puppet theater.
If I had a doorway I could use safely and easily, I'd just put up a tension rod with fabric draped over it, but I pretty much need a tabletop model. The puppet theaters I'm trying to decide between right now are this one from Pink and Green Mama and a version of this Shoebox theater. I also like that they are easy to pack up and store, which is a major plus for my classroom/office.