If you know how to tune, you can get rid of nearly all noticeable overtones by taking advantage of phase cancellation.
Otherwise, I've heard that those little gel letters are made of the same material as moongel, but cost significantly less. To be honest, if you MUST use muffling, I'd just use moongel. Do NOT use tape. This is not the 70s. All it does is leave nasty adhesive residue on the hoops and heads, and it looks amateurish.