Just let the mask sit on the top. Will be fine.

If you want to get real picky, make a 5x5 by taping some totally exposed and processed negs together with silver slide binding tape that is super thin. now cut a hole where you drop in a 4x5 neg.

Before I made my 35mm one side glass, I used the 4x5 double glass with a mask on top and bottom outsides about 2mm bigger than the film image area. It was ok, but a pain to work with as the neg strip had to be oriented at an angle to clear the locating pins that were centered. They sell it as a universal glass carrier so 4 locating pins should be used to accomodate film strips.