Literally finished this one just now
it's been a long time in the making. Originally it was just a 4'x8' enclosure with a concrete floor. Now look!
-NIC condo with carpet-covered shelves, with a spinning hay ball on the top floor,
-Big, permanently open cage lined with woven timothy mats and filled with (kiln-fired pine, which removes the harmful aromatic chemicals) shavings, and a hideaway box (that has a fleecy pet bed inside) attached,
-two litter boxes, one a lipped cat litter and the other a DIY enclosed litterbox from a storage bin, both lined with newspaper, filled with shavings, and topped with hay,
-enough toys and chewables to play a hidden object game
-soft pile carpet for their tender footsies
And I try to, once a day for at least an hour, open it up so they can have full run of the basement.
(And speaking of which, they are directly below a vent, I have a space heater running, and there is a broad spectrum light hanging above them on a timer to supplement the weak sunlight from the tiny basement windows. Environmental controls are part of a good enclosure!)
(And by a long time in the making I mean 8 years. Though this enclosure began more like 6 years ago, they lived elsewhere before that.)