Just use a large storage bin for their litter box. These are so tall all the hay stays in. If your bun is small and doesn't love hoping in and out, simply makeshift one or two stairs to the litter box using a hiddie hut or anything else that works. Even if they jump in from a level flush with the top of the box, they still have to jump the distance out which is when all the hay drops off of them and back into their litter box.
I covered the top of my bun's hiddie hut or "stairs" with large sheets of sand paper I simply glued on with Elmers. This way they have plenty of grip so they don't mind the extra hops at all - not to mention this files down their nails so win win!!
You can also use a cooky tray over the litter which keeps the bun's from sitting in and getting tangled in piles of hay so they don't track much hay out.