Nice. I made something similar recently:
View attachment 331122
It's a 2-piece design like yours, but with a hinged lid. I included a recess for a little cylindrical magnet in the baseplate and a metal plate in the lid so that it snaps shut. For the hinges I generally melt plain iron nails (1.8mm diameter) through the hinge holes.
Since sheet film is quite stiff, it sort of works OK but for critical sharpness and big enlargements the sag can be a problem. A tensioning system or a glass carrier is preferable for those. These 3D printing thingies work in a pinch and for non-critical stuff.