I took a litter box, put nothing in it but a bit of hay and some "chocolate drops" the bunnies produced and they jumped right in and started using it. I read that they like eating and pooping at the same time.
I can't remember if they were fixed at the time, but they are now. For litter I use Feline Pine. I have to secure them in the corner though, or they still try to tip them over.

I can't remember if they were fixed at the time, but they are now. For litter I use Feline Pine. I have to secure them in the corner though, or they still try to tip them over.