If I am not mistaken, the masking attachment for the older Omega enlarger works. I think from the D2 era. You could probably find one of those.
I use rubylith tape on my glass carrier. Works fine but over time I suppose it would be more expensive if you are changing it a lot. I only went this route because of 6x12 negatives.
I've also made masks out of aluminum flashing and covered them in telescope flocking. I cut them to the size of the top glass and just lay them on the glass with the flocking against the glass. That would be the easiest way to go and you won't have to beat your head against the wall trying to find something that may not exist anymore.
I never liked masking attachments. If you are slightly off with one of the blades, the shadow will soften the edge of your print. Usually you don't notice it until the next day. Ask me how I know that... Lol.