Thanks, I found it now, the one I was referring to is like
this. I don't actually slap that much, I play metal, 'clang' style. Basically every attack hits the 21th fret with a lot of force so my 21th fret is pretty well chewed by years of clanging, which is why I'd look into a solution such as this, because my 20th fret seems to get some damage too.