*SCREAM* NOT AGAIN!!!
Mowgli I busted you out once but I don't think the warden is going to fall for it again!
I think you need a choice of a clumping litter and if that doesn't do it then a vet check just to be sure you deserve the sentence you are serving!
As much as I hate to say this! The only other thing you can do is what your doing now. When you can't keep an eye on him then back to the cage or bathroom with his box. Kind of like retraining. If that doesn't work then I don't know! You could check with the vet about medications that can help.
I really would try a regular clumping litter first before I did anything else. It would be a nightmare to have to use regular old clay but I guess you should try it too. That would probably be his favorite!!!

*CLANG, CLANG* (tin cups on bars) FREE MOWGLI *CLANG CLANG* FREE MOWGLI!!!