Wait, "the" litter box? Multiple rabbits and one litter box?! Oooh, mine wouldn't go for that, either
I have two rabbits, and they currently four litter boxes (two on each floor). When they were upstairs rabbits, they had three litter boxes -- any fewer, and we'd have problems.
Rabbits are very social, so if one's in a litter box , the other will want to go to the bathroom as well... and he/she needs their own litter box.
Also, the litter boxes need to be large & spacious (rabbits able to turn around within the box), with tasty hay (refreshed twice a day), and in their spot of choosing (you got this one covered).
Yes, it's ridiculous and and a lot of work/litter boxes... but it's what works for my guys.