Here's a drawing of the box, showing the walls, the base and the cut out slot.
The sculptor sent photos today, and I'm not sure if there's going to be enough space from side to side for a two-piece box. It may be that I'll have to have a flat piece made with a slot cut in it that will attach to the top of the box.
I think I need something will walls because I'll have a plunger or something like that going through the wall or base of the box to hold the blade in place with friction, and then when the blade is extended, to lock the blade into place.
I'm thinking that 22 gauge steel would be sufficient.
The two little sections of wall on the front are to act as stops where extra pieces of metal on the left and right sides of the tang of the blade would come to rest when the blade is extended. The slots in the bottom box and the top box (or flat piece) would be for pieces of metal on the top and bottom of the tang, and would also stop the blade when it's extended. The extra pieces of metal riding in the slots and the closed box would serve to keep blade movement to a minimum when the blade is extended.
The blue piece in the diagram below would hold the retracted blade in place by friction. When the button is pressed, the blue piece moves out of the way, allowing the blade to slide out along its tracks. When it's extended the blue piece is dropped back down (or inserted from the side) behind the slot in the tang, holding the blade open.