Take a one-piece pajama, the sort with long arms and legs, but without feet, that has a zipper running from one leg to the top, and put it on backwards. It's completely tamper-proof, easy to "service", and ought to be equally comfortable.
We have tried many of the other suggestions (diaper backwards, onesie, onesie plus some kind of trousers, "duck" tape, sleeping bag), but they were all found out after a short while. If he gets the backwards pajamas off, he'll be ready to join a circus.