I remembered (and checked) that yesterday I said it was my right shoulder that hurt, but it's my left shoulder. I'm not just losing words and forgeting things, I'm losing directions.
Last night Spirit got up on the castle by herself and then spent some time on the recliner, ending up in my lap. She started to pee on the towel, but I quickly moved it out and put the litterbox on the shelf of the end table and after looking closely at it, she used it for the rest. So maybe the litterbox needs to be that much closer, but then the towel would move farther from the recliner, and I don't know where I'd put her food and water.
Hmmm, I just searched again and found a triangular litterbox that will fit on the shelf of the triangular end table and bought it. I can put her food and water on the other table's shelf. I hope this works because I already have an extra litter box.