We may have found the solution to the Bart "stinking outside the box" problem. It appears to have been working pretty well for the last two or so months.
Back when we had Bart and Theo, there were problems with Theo peeing over the edges of the litterbox. This was because he always backed up to the edge of the litterbox before starting... and he was a big cat. I first solved this with the "Kitty Butt Shield" -- an invention I came up with involving basically extending the walls of the litterbox upward to prevent this from happening. It worked pretty well. But then we simply bought a big plastic bin for a litterbox. We dump a ton of that scoopable litter in it, and it seemed to work well.
For some reason we went to replace it one year and I couldn't find one the size I wanted with reasonably short sides that the cat could still hop in. So I cut one side down to the right height, leaving the rest nice and tall.
Bart eventually took to peeing in front of the litterbox at times, so I moved the box right up to the kitty bathroom door (a hole in the wall behind the toilet downstairs leading into the space under the stairs where the litterbox is). The tall walls would deter him from going elsewhere under the stairs to do it, and the proximity to the door meant that once through the door he would be IN the litterbox. No choice.
So he started peeing outside the door by the toilet instead.
He was also leaving us solid treasures near the bottom of the stairs.
At any rate I decided, after trying cat-scrams (with some success, but their effect wears off on him after a while) and various litterbox positions like this and even a bit of flashing to eliminate the "corner" outside the litterbox door that he favored -- each providing limited, but temporary improvements, I noted that when he did this there was usually a large, wet clump in the litterbox RIGHT INSIDE ... that he would have to step in to get anywhere else in the litterbox.
I also noticed that, in a most un-catlike fashion, he wasn't burrying stuff like cats do, and hadn't been for some time.
So I decided to do two things. Vicki thought I needed to move the litterbox back so it wasn't right next to the door anymore. Like he felt a bit crowded in there. So I did. But he soon peed right in front of the litterbox againSame problem... big clump right at the part of the litterbox where it was easiest to get in. So I cut the entire front side of the box down to that level. He seemed to like that better.
And I switched litter. Actually it's the cheapest scoopable stuff available at Walmart now. "Special Kitty". Works fine. In addition, I added a "foyer" - basically, a low clear plastic container that is about as wide as the front of the litterbox that fits neatly between the litterbox and the door that he has to walk through to get in the litterbox. This is in case he decides to occasionally pee in front of it to show his displeasure. At least we can just rinse it out and put it back.
Well, that seems to have done the trick with one caveat: We have to clean his litter every 48 hours, pretty much, or he'll pee in the foyer. But that's much easier to deal with now. He's burrying his stuff again (I think he didn't like the litters we'd been using. Could've been the scent, I don't know). And he fills the thing pretty well in 48 hours. That cat drinks a lot. And he's not diabetic. He just likes water. He's fascinated with it. He wants to lick it out of the tub after we get out. He wants to drink it from the bathroom sink. He loves to watch it swirl down the toilet. And if there's condensation on your glass, well he'll be there to lick it off.
So at least we've reached some sort of truce on what is acceptable to him for a litterbox. As long as we clean it every 48 hours, use that unscented litter, and provide him easy access to hop in the litterbox from an entire side of it, he appears happy with it.