training cat to use toilet?

Alright I’ve asked this q one other time and got a bunch of yahoos and got no help except for a couple of good answers. My cat Kiki is about 23 pounds and 3 1/2 feet long all streched out. He is on diet food and is actually just a big Buy Propecia cat. Anyways we have the biggest litter box ever made and he still tetters on the edge and just dangles his but over the litter. He also doesn’t cover his poo. We figure he has the technique down we just need to switch madiums. Has anyone successfully toilet trained an older cat? I would fill the bathtub with litter for him, but i still need to bathe.