As a 3D Printer and Designer I see a lot of people asking questions about electrical connection boxes and faceplate covers.
Something most people don't seem to know is that everything absolutely everything involved in the electrical world is (in the U.S.A.) certified by the organization "Underwriter Labs".
If your part is intended to be used in an electrical application, it needs to be UL certified. Otherwise your client will not be able to have a successful insurance claim in the event of a fire. Producing such parts can also open you as a producer of the part to potential lawsuits.
When it comes to in wall boxes, just use UL certified boxes and deal with it.
If someone's insured property burns down and during the investigation it's found that non UL certified items were used, the insurance co. can use this as their golden parachute and deny any related claims. After that, the property owner is going after the people that produced the part, and the defence of "You used it wrong," won't work as you probably can't afford to buy a judge that week.
As for faceplates, simply cover a UL certified faceplate with your embellished plate. Once you are outside the confines of the box, you can do whatever you want.