Is it a Mk2 or a Mk2a? In this case, I'm asking if you have the completely round stem or one that has a segment cut out of it?

Now, what kind of knob do you have? There were three styles: the original had a hub that was larger than the stem and had fine vanes that connected it to the stem; the replacement had a hub that narrowed to the diameter of the stem and used a small O-ring to make the mechanical connection; the third had a hub that was keyed to fit the segmented stem.

I almost immediately had problems with the original one. Replacements were mailed out for free, and I have never had a problem with it at all. Or the black Greenlights replacement buttons I got.

Anyway, what I'm getting at is that the correct solution is to replace the knob, and adding material to the stem might prevent you from using the correct solution in the future. (I don't know how hard it would be to get that epoxy off.)

If I were you, I'd modify the knob before I modified the stem. Assuming you have the original knob, it should be pretty easy to find a rod with the same diameter as the volume control shaft. Put that in there, fill up the hub partway with epoxy, let it set, trim off the rest of the vanes, and then put it back on with an O-ring. Make sure you have some sort of release agent on the forming rod: vaseline, maybe?