Something built along the lines of a bathroom medicine cabinet might work.
A shallow display box with a hinged front door that can be opened 180 degrees, perhaps with a poster of a cutaway cartridge on the inside with all the parts labeled.
The shelves would be about one inch thick with holes drilled about one half inch deep to hold the cartridges. The shelves would be thick enough for the cartridge names to be displayed somehow on the edge of the shelf.
Put a handle on the top and maybe removable feet on the bottom that are wide enough to stabilize the box when it is on a display table so it doesn't tip over.
You could probably pick up all the materials at Lowe's or Home Depot and your son can build the whole thing himself with your supervision.
