Bob built ours - and I helped some! We used 4 x 8 sheets of plywood. The sides and the bottom are 4 x 4, the top is a frame with wire fencing so you can see in but mom can't jump out unless you leave it open. We cut a 'door opening' in one corner and attach a wire crate, then use a small piece of wood to create a low barrier that mom can step over, but pups can't quite get over until they're able to climb better. This is mom's hideaway from the babies. We put a pig rail up and also set up one corner with a low L shaped barrier to keep pups in one section until their eyes are open. We put linoleum flooring in, and as they get more mobile, add artificial grass for better footing. We actually have 2 boxes, one we made with hinges on the sides so it's easier to store and move (the solid one is really heavy - maybe casters/wheels on the bottom would help? I know I'm not describing this well, but at least you can visualize it a bit. When we set them up for this year, I'll try to get a couple of pics and post them. We use a heat lamp for the first few weeks - on a tripod, so it can be adjusted easier.
Pups are well protected from drafts and curious Basenji house mates, and I can climb in with them! Aaah, puppy breath!