1) you can put shredded wax paper in the box. It will not absorb urine, but will give her something to scratch around in.
2) Little Kim had urine crystal inspite of being on the c/d food. The vet told me to allow her NOTHING else to eat. no treats, nothing and then retest her. It worked, so be careful to give Luna only the prescription food. The vet said, for a cat with this problem, even a little bit of something else could be harmful.