I have tried a covered box. She did not seem to like it much. It was one of the first thoughts I did thinking maybe the dogs were bothering her. Poop incidents increased, and them decreased when I took the cover off.I know you're probably fed up with spending all of this money on litter boxes (like I would be). But have you tried the covered boxes that give privacy? (Also called Buddah boxes) Maybe it's not the length of box but the amount of privacy that could make a difference. With the box covered, the "walls" around the box could emphasize to her the importance of going "inside" the box.. not just around it. Plus it could give her a sense of control/own space if this is a psychological issue (especially if the stray cats are stressing her).