Glad to hear you are intent on doing the primary decoupling at the base of the speaker stands since you need to bolt on. I'd consider a thick soft towel, undercut to not show. I wouldn't use anything at all that might leave any type of residue or mark, I doubt a cotton towel would. With a thickness or two of towel, you can crank the bolt down a bit and still have some decoupling via the compressed towel, as opposed to pretty much zero decoupling with a piece of paper, e.g.
Still, if you can, test all of the items just to see if there is an audible effect.
(I've been stuck bolting inexpensive monitors as well in prior years due to cats... "No-they are not cat-stands!")