Cat urinating outside box, any suggestions?

If you know me, you know that I'm a vet tech and cruelty investigator, so I know a thing or two about cats! I have 5 kitties right now. The newest one was semi-feral, at one point, she was a housecat, but her family moved away and left her behind. So I took her in. She has a heart problem that we are trying to stabilize, so she hasn't been spayed yet, I'm sure this has A LOT to do with the problems I'm having, but spaying right now is not an option. There are no medical problems. My vet and the vet I work with are out of suggestions for me, I've tried everything from spray bottles while she's doing it to switching litters.....I'm at my whits end. Any suggestions would be greatly appreciated!