If you use a piece of sandpaper the full size of the pocket floor, you’re not changing the angle. You’re merely raising the neck about a hundredth of an inch.Ok. My concern about using it on the floor of the neck pocket is that then I'm changing the neck angle. If I wanted to shim it in that way, I'd be more comfortable using a full pocket shim.
Would there be a better material to use for a side shim?
Use ultra fine black emery paper and stick it down with CA glue or double sided tape, etc.