For the past... gosh, probably about nine years, I've been housing my rabbits in 4x4 dog playpens when indoors, and bedding them on hay. It was messy, expensive, and annoying, yet for some reason I kept doing it under the rationale that my old bunnies surely would never be litter trained. Well, finances have finally compelled me to find a cheaper solution, and now I'm kicking myself for not having done this sooner! I threw down some fleece, set up some litter boxes, and lo and behold: it's working perfectly! Aside from a stray poo that can be swept up, the buns are using their litter boxes beautifully, and it's so much easier to just dump out a box every other day than do a major weekly cleaning on the bedding as a whole - not to mention cheaper. And as for my concerns about old dogs and new tricks - hah! The gray and white bunny you see pictured here I obtained as an adult and have had for seven years, and she picked it up right away.
Sorry for the lousy pic; it's in my basement and the lighting is lousy for photos. These guys are mostly out in 10x12 enclosures grazing during the day, but in poor weather (which is common in Alabama) or temperature extremes they come downstairs.

Sorry for the lousy pic; it's in my basement and the lighting is lousy for photos. These guys are mostly out in 10x12 enclosures grazing during the day, but in poor weather (which is common in Alabama) or temperature extremes they come downstairs.