I recently tested out a product called the HECS Ball, which is basically a pom-pom that’s been specifically created for practicing your racket skills against a wall of your home. It was invented by a badminton player and their website has videos about how to practice badminton indoors, but the same idea can be applied to squash.
I found these balls were quite fun to hit against the wall in my office — especially the dark gray one, or “#1” size. The #2 size is also pretty good. I have them on my desk and sometimes will take a break from work to hit a few shots against the wall. Obviously you could hit a regular squash ball against the wall (unlike a badminton shuttle) but part of the selling point of these HECS balls is they do not leave marks on your wall.