I've been applying an industrial design eye to the box.
Some thoughts.
It's a big investment to make something like this, it seems worth doing it right.
As I modeled it, the box will be "floppy" - the back is just a 10mm panel. If I were really making it I'd make a frame behind it 5-10 cm thick. One lovely feature of boxes is being able to wheel your victim off to a new location, so making the box sturdy enough to handle that is something I'd definitely want.
Laying the box down is useful lots of times. But then it's really awkward to get into the box, or else the box is on the floor and the top has to kneel down. So I designed a trestle to go with it. With someone in the box, the center of gravity is high enough it can easily be tilted back. There's a square frame on the back of the box. The lower rim of the frame catches on the bar on the end of the trestle, and becomes a hinge. Since the center of gravity is somewhere near the hinge, it should be easy to tilt the box back horizontal. The rest of the square frame becomes a guide to make sure the box stays on the trestle. Once fully tilted the box can be slid back fully onto the trestle.
Here's a pic of the trestle arrangment
The horizontal boards to make adjustable dividers are flexible but don't allow for precise adjustment. I'm thinking of replacing them with some other system.
The holes in the dividers need to be positioned so the user's weight isn't on the divider edges when the box is laid down. And I'm playing with options for padding in the back.
Some threaded inserts in the dividers would be a nice solution for mounting toys like a fixed penis gag or fucking machine.
Having separate dividers means you can do variations - hands at sides, hands beside head, hands in with torso, divider below breasts with arm holes for breast play.
While stuffy is part of the experience, do make sure there's enough ventilation to not be a hazard.
What else should I be thinking about? I'm having fun just thinking through all the details of my box.