I used to have a corner litter-box, and while she'd use it consistently, her butt would hang off the edge and make a mess in the rest of the cage. I recently just switched to another litter-box; I bought it about a year ago, but realized it was a bit big for her cage. However, I'm switching her cage out for a condo this month, and wanted to start the transition slowly. So the first aspect of her habitat to change is her litter-box.
(As well as the relocation of her hay racks, so that they're next to the box.)
I bought a litter-box for ferrets, because the cat litter-boxes seemed really bulky, and they just looked enormous on her (she's 4 pounds). Also, ferrets apparently have peeing habits that require them to have high walls, which I wanted for eliminating any opportunity for her butt to hang off the edge. This box also features an easy-to-access entry, wide and low. I'm surprised no one's mentioned ferret litter-boxes here yet! Though this one may be a bit small for a medium-sized rabbit.
Sorry for the bad photo quality. DX Misplaced my camera, so I used my laptop's built in one.