Me and a friend just made one, 4x8x6. We used 2x4 to frame the floor with a half inch peice of plywood on that. For walls we used 2x2 with luan on the outside. We went simple with the roof and just used a flat peice of plywood with metal roofing on it. Added a couple of small pexiglass windows and insulated the whole thing with inch and a half styrofoam.
You might notice the lack of holes, we were saving the best part for last. We're planning on just using a jig saw to cut them out and hinging them.