I used a large size greeting card which I attached to a square wood floor covered with scrapbook paper. The walls are insets made from foam core board, also covered with scrapbook paper. Butt one wall on top the other to glue.
I put wood strips at the top and sides. Acrylic pieces can then be fit. The side pieces are glued to each other and to the top.
Courtney btw, runs the great blog site, Haunt Jaunts. Check it out for some neat info on haunted places!