Cleaning cat urine with bleach can be very risky as the combination of ammonia and bleach produces something like mustard gas and it can burn the lungs.

I've read that Dr. Bronner's Sal Suds with a little vinegar in the water will clean cat small. Dr. Bronner's soaps are extremely concentrated and a little squirt in a pail of water works well. Maybe you could stir an old shirt into the litter box and see if it takes the smell out.

You can usually find Dr. Bronner's soaps in organic stores.