I have made a mask to use in a 4x5 carrier by cutting the right size hole in a 4x5 sheet of film and then taping the negative down with tape.
You can make a hinged negative holder with two pieces of black matting and a piece of tape, but you always have to be careful to get it put in the right place over the lens.
You could also tape the film down to a piece of glass and mask it off and slide it in there. But again you have to be careful about alignment.
Best just to buy the Beseler ones.