I don't think I'm gonna be a big help to you in that Forest is older, fat & lazy. LOL.
He is very regular about doing his "job" outside everyday. I take him outside to bask for a few hours every day always in the morning & every afternoon. He goes every morning & every afternoon as long as I keep his time outside pretty regular. Twice a day, outside & he's darn good about taking care of business then.
I don't skip an outside time & he, in return, doesn't leave me any surprises. He's a great boy & I'm so lucky.
Then again, Forest is almost 8 years old, knows where the door is when he's done & can be left alone for short periods of time.
Thought of a kitty pan? Forest's previous owners said he used one, although never has since he's been here. It's all outside for him now.
Maybe that would work?