Pipper was litter trained very fast. She stays in a large Super Pet cage when we are asleep / not at home. When I first brought her home, I observed which corner she urinated in, and simply placed the litter box there. Simple as that. However, she still does leave pellets when she's out of her cage, which I've yet to learn how to deter!
If your buns urinate in different corners, you may need to purchase two litter boxes for their hutch. Once you give them a litter box, they'll likely only urinate in there!