My Durst m670 bw has a negative holder that looks like a book (sort of), where the top-side is newton glass and the bottom is open.
Then you insert the negative mask, 6*6, 6*7 or 35mm, depending, and close the "book" after also placing your negative in there.
The negative masks are made of metal and are almost exactly the thickness of the plastic used in DVD covers.

So, since I was missing a 35mm mask at the time, naturally, I made one by cutting out the plastic of a black DVD-cover to the correct specifications of my negative 6*7 mask.
(the only difference is the hole in the middle)

Then i made the "hole" for the 35mm negative with a sharpie.

- Actually I was trying to be as precise as possible, but the hole was way too big, creating a wide, black border outside the negative.
- So i used black electrical tape to close the gap to fit more with the 35mm negative

And voilla!

The good thing about this method, is that you don't have to ruin your real negative holders, as getting replacements isn't that easy these days.
I also made a dirty border for 6*6 frames out of cardboard, but this has too much paper-fiber on it, looks ugly, so i may just make one out of plastic for that too ^^

Mileage and enlarger-systems may vary, just be creative with some scissors and a knife and some tape, and you can probably make something that works