Hopefully that will do the trick and Thumper will be able to get into the box alright. Does she use all the litter boxes or does she favor just one? Some of my cats favor a certain box and others will go in whichever is closest.

Hopefully it won't come to this, but if switching to lower boxes doesn't work you can use puppy pee pads. (You know what I'm talking about, right?) I had a cat that had a neurological disorder, she had extremely poor balance, she couldn't stand in litter at all, so she used the pee pads. A friend of mine used them for an old cat too (over 20 years old). She put them right next to the litter box, the old cat would use the pee pads and the rest of the cats continued to use the regular litter box.