Originally posted by A.Wise:
...I'm looking for a deep shadowbox with a door on it...
The clear acrylic guitar case from Gemini/Showcase is a very good presentation, and includes the mounts for the guitar.
But if you must have a wooden box with a door, you can build it. Almost no woodworking is involved.
Framerica "Boxers" work like ordinary frames and may be stacked to any depth. Just buy the chops, glue/nail each extender frame together as usual, and stack all but the top frame using glue and staples. The shadowbox sides should be glued to all layers of the stack, which will reinforce the assembly.
The top "cap" frame holds the acrylic glazing, which is better than glass because it is lighter & shatter-resistant. I suggest Optium Museum Acrylic for deep framing -- it is almost invisible.
Use a piano hinge on a long side. If it is hung vertically, that would be on the left. If it is horizontal, it would be on the top. You can make a gasket out of Volara foam tape if the customer wants it dust tight.
If the piano hinge is on the top edge, gravity will hold it closed. If it is on the left side, use a few rare-earth magnets to hold it closed.
One question you didn't address: How will you mount the guitar inside the case, so that it is properly supported, unharmed, and removable/replaceable?