Moleskin is the old standby for building up a comb in increments to find out just what you need, and can be removed without issues.

I have a tendency to use things like that, that can be easily removed, without damage, after I tire of the gun, or pass over. eek

I don't much care for permanent "fixes" to old vintage guns, anymore. The next owner may have a longer neck than me, or longer arms, or fatter cheeks, or ....... ad nauseam.