Mat board has been used for custom sized or improvised negative carriers approximately forever. A layer of packing tape, carefully applied to be wrinkle free, will help avoid negative scratching. Strips of straight tape can act as rails to guide the negative strip, to ease getting the actual image area aligned with the mask opening.
Another option is glass -- Anti Newton Ring (or the anti-glare glass from a better picture frame, much cheaper but less durable than acid-etched ANR) is preferred for the upper side, which contacts the film base, and strips of tape can serve as both negative guides (between the glass layers) and frame mask (bottom side of the lower glass, so it doesn't interfere with negative positioning. Once again, this is great for sizes that are hard to find -- 6.5x9 sheet film, for instance -- or oddball film sizes, or those with offset frames (828, 126, 110, Minolta 16, etc.)