I'd understood that they like to have cover while they poo, and they like corners. Almost by accident, my indoor cage has ended up with a small tray in a corner with a cloth cover over that corner of the cage (an old hand towel pegged over) and that's actually worked and become the toilet, mostly. They do the majority of their business there (not all of course!) so it does help with the cleaning.
The tray is lined with either a puppy pad or newspaper and then there's a pile of hay on top. It's funny watching them jump in, then kind of reverse their bum into the corner and wiggle a tiny bit, you know exactly what they're doing!
But yeah it's not the only place they go, it catches maybe 50 percent, with the rest spread evenly around.