(grrr… my computer keeps messing up when i try to post, let’s see if this works.)
i would try alternative bedding in the cage. either a towel or inexpensive rag rug that can be washed, or a straw or grass mat that can be chewed and replaced occasionally. i assume your cage is not wire bottom, so there’s no need to have something down to protect her feet. what you may find is that even given an option of a towel or grass mat, the bun will probably just lay down on the solid surface for much of the time. buns are weird like that… lol. you do want to make sure she is on soft ground sometimes to prevent sore hocks. also keeping nails trim and making sure she is not overweight will help with that. the good news, you’ll save money on not having disposable bedding in the cage, plus it will be much less messy and will allow you to truly see how she is with the litterbox.
onto the litterbox… my first rabbit Stepehn was a litterbox pusher. he wouldn’t dump it, just slide it all over the place. we had one of the large high side corner boxes for him (and for Meadow now) my husband drilled a small hole in the top corner of the high side and we would loop a ziptie thru it and around the cage bars to keep it in place. the only downside… you have to cut the ziptie each tiem to clean the box, but they’re very cheap to buy, so we would just have to loop a new one thru every 2-3 days when we cleaned the box. most importantly, it worked.