If I read the charts right, the FM2 takes the 19 mm. thread, common to the F and F2, etc. If the lenses are the same as the old F ones, they are all metal, and the original F rubber eye cup fits over them. That is, as far as I know, the standard rubber eye cup both for the Photomic F and for the DG-2 magnifier, etc. I have an eye cup on one of my F's., but the others are bare. Glasses scratching is not an issue, because the eye point is not high enough for glasses anyway.
If the newer version has a rubber ring, you will probably have to remove that to put on an eye cup, but you also may not need the eye cup.
The accompanying picture shows an F diopter with an eye cup on it, and the same type F diopter with no eye cup. The eye cup just stretches over the flange, and is held tight when it's screwed in.
In front of these are a pair of HP diopters. The left hand one has a small rubber ring glued to it. The right hand one has a wider metal flange, and the rubber ring (behind it) stretches over it. I do not know which versions are meant for which cameras or which one of the two version is newer.