Is the negative part of a strip of negatives? If so, you probably don't need to worry about it - just crop using the easel.
What type of enlarger and negative stage are you using?
“Photography is a complex and fluid medium, and its many factors are not applied in simple sequence. Rather, the process may be likened to the art of the juggler in keeping many balls in the air at one time!”
Ansel Adams, from the introduction to The Negative - The New Ansel Adams Photography Series / Book 2
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!
Yes you can use the 6x6 but if for some reason that shines too much extraneous light, black matte board works great for custom neg carriers.
Yes, that will work well. Another possibility is to use a 6x6 frame of fully exposed and developed film, and to cut the 6x4.5 window out, then sandwich that with your negative. Do the final cropping with the easel.
Your baseboard and easel should be yellow, red or black, which will not reflect any light that your photo paper is sensitive to. I crop 4x5 negs down quite a bit when needed and never have felt the need for masking the negative.
Now, if you have bright white baseboard and easel, you might want to consider it...
I want to print 645 and my school only has 6x6 negative carriers. whats the best/easiest way to mask it to work?
I was lucky enough to find thin black plastic sheets; only 1mm thick and flat black,which I used to cut myself several masks for my L1200 negative carrier; worked great, but I don't remember where I got the material;it can be easily cut with a carpet knifeinto any shape; also great for making custom dodging tools.