Are you using a “regular” rectangular litter pan? Is it big enough? I like to get ones that are at big enough for Leroy to lay down in (he thinks litter boxes are the best place for “chilling”) without having to lay on the soiled litter. You might want to try a pan designed for corners, they have a high back. I have this one for his cage, where there’s less space: http://www.petco.com/product/5300/PETCO-Small-Animal-High-Corner-Litter-Pan.aspx

You could also try a large cat litter box with higher sides, but if the sides are too tall, you’ll have to cut out part of one side so your bun can hop in and out (and then make sure there are no sharp edges so she doesn’t scratch herself).

For a quick fix, you could put the litter box in a cardboard box to create higher sides – cut down the front area to allow easy access. Or else you could just cut the box in half and put the cardboard guard at back of litter pan. If she pees over the edge, the cardboard will collect the pee and odour. Its easy to replace and cheap and the cardboard it safe for her to chew. Otherwise, get a high sided pan/even storage containers work well.