"Dog Proof" may not be as accurate as "Dog Resistent" or "Dog Challenging", but so far it has worked well in our household. In fact, our dog has given up on raiding the litter pan so I consider it a complete success. We have large retrievers who are much too big to fit through the doors, although small breed dogs could walk into the litter box just like a cat. By altering the size of the door openings, however, this design should accommodate many cats & dogs. Another feature of having a box with a "foyer" is most of the litter on your cat's feet will stay inside the large crate as they exit.
Let me show how to build this inexpensive litter box for your cat...and not your dog.
Step 1: Stuff you'll need ...
30 gallon storage crate
18 gallon storage crate
The 2 crates need to be similarly shaped so they seat nicely together.
1 1/2" hole saw & drill
4 wood screws
Small piece of mat or carpet