Thank you for rescuing this cat. I have a few suggestions for you. A dog litter box may work. They are usually lower in height and wider than a cat litter box. If you are unable to find one you could use a low plastic storage container like the ones designed to slide under the bed. Another option is to cut down a cardboard box to an appropriate height then insert it in a plastic garbage bag then fill with litter. For litter choices I would try a pelleted litter rather then clumping so it wouldn't stick to his fur if he falls or lays in it. I also think the urines drains away faster/better with the pelleted litters. I would take him to a vet or groomer and have a sanitation type clip done, basically to keep his hind end clean. Then keep a package of bathing wipes for pets to clean him as needed. Without the extra hair he will get less messy and be easy to clean. Due to the amount of urine output you might consider having the vet check for kidney issues, diabetes, etc.
Once again thank you for rescuing him and best wishes for making this work .