I made a nice little puppet theater from a cardboard box. It only required three cuts and some strong tape, no extra cardboard was necessary.
Materials: sturdy cardboard shipping box, utility knife, metal ruler, a pen or marker, strong tape such as clear duct tape.
Step 1: Sealing the Back Wall
Determine which side of your box is going to be the back wall of the theater; I used the side that had been the top of the shipping box.
First, tape down the flaps. Lay the long side flaps down first and tape them together at the seam in the middle. Then tape down the two shorter flaps, with a little bit of extra tape over the edges to keep them tight. I ordered them this way so that when you look inside the theater, the back wall is smooth and you could paint a backdrop right on the wall.
Step 2: Cutting the Legs
Flip the box around so you are working on the bottom of the puppet theater. Using a nice strong ruler, cut through the box along both of the long sides about an inch and a quarter in from the edges. Then measure and find the middle of the box and make another cut through the box, perpendicular to the first two cuts. This makes two flaps which will be the legs of the theater.
Step 3: Opening Up the Stage Front
Once you have the bottom of the theater opened up, open up the front of the theater. Push three of the flaps inside the theater: push one onto the ceiling and push the two side flaps onto the walls. Tape these three flaps to the inside of the theater. Take the fourth flap and fold it down outside the theater.
Step 4: Making Feet
Turn you theater upside down so you are working on the bottom of the theater. Pull up the two side flaps you cut out in step two and pull up the front flap you found in step three. These will make the legs of the theater. The front flap will have to be angled back a little but bring it so it meets the side flaps. Mark the side flaps where it meets the bottom edge of the front flap and score it there. Do not cut all the way through. Bend back the side flap where you scored it and reinforce the score (when it is folded) with tape to create a foot. Now the three flaps should be the same length.
Step 5: Taping the Legs
For the last step, run a continuous piece of tape along the bottom of the three flaps that make the theater legs. This will hold them together so they are sturdier.
Flip it right side up and decorate as you wish.
A few more photos on flickr.