You always can take a simple file folder, and trim out a 645 space to serve as a mask to block stray light which might reduce contrast. Then cut down the outer part of the file folder and punch out round holes where the metal pins of the 6x6 carrier would protrude through the file folder. And black out the file folder with black felt marker to cut light transmission to minimums.

I had a 6x6 enlarger as a kid, and did exactly what I described to make a 135 format mask...not enough money in the budget of a 14 year old to buy a 135 format neg carrier!