I was thinking about that with black matt board or cutting away a 6x6 metal carrier I had to fit 6x9 with a dremel. The problem is negative flatness with something the size of 6x9, the universal carrier is ideal because it has two glass surfaces to press the negs flat. Anti newton on top as well.

I can rip out the carriers masks But removing the sliding masks will reduce the functionality of the carrier as i print half frame, double half frames, 35mm, 35mm black edged, and 6x6 with the use of masks.