The petsmart litter boxes are so $$$ I bought one and wish I would have thought of this before spending the money. Maybe you can buy a regular size plastic toat box?? Not sure what everyone calls them. A plastic storage bin. Leave the longer sides there (so if the leg is lefted the pee wont go everywhere. Cut out the one shorter side for him to go through. Most plastic storage boxes aren't much. I just bought a few and they were 5 bucks each so maybe this could work for you.
Good Luck