Just wanted to chime in here too because I was just going to share a tip that I found worked for myself.

Shelf liner, like Christopher mentioned. It's cheap, you can get practically anywhere, and it comes in all different colors. It's a very inexpensive solution.

I have been using shelf liner underneath my sewing machine foot pedal to keep it from sliding around. Then I saw a tip somewhere about using shelf liner to keep the guitar from slipping off your leg in a Classical position. I tried it and it works so well! I also use a foot stool to get my leg at a proper height and a strap to keep the guitar close to me. One of the authors/guitarists of a book I have highly suggests the classical position for any fingerpicking, which I'm working on now (with my good hand!).

When I can return to playing with my fretting hand also, I plan to stick with the classical position for most everything (hopefully) as it's supposed to be much better on your wrists.