I am going to post the images of my first build here, along with a few notes. I need to redo these for the redo using the led strip lights instead of the rope lighting.
Here is the assembled lightbox from the front. It looks just like a standard spotlight displays frame here
Here it is from the side. I used pine 1x2 lumber and painted the outside flat black. The backing is a piece of masonite shower liner with a glossy white coating on one side
Here is a front view with the frame open. That is 50 feet of led rope lighting, held in with cable clamps and clear packing tape.
A corner detail, showing how to mount the spotlight frame to the box. I forget the exact dimensions of the frame, but they were chosen so that the side to side mounting holes were centered on the wood frame. Note that the inside of the mounting box is not painted.
Another detail, showing the cable clamps and packing tape used to secure the rope light tightly against the back of the box.
Clamping the poster in place between the 2 layers of the PET-G of the spotlight frame
The box lit up in a dark room. Note that the strips can be seen. The effect is much less obvious in real life, but this is a reason to go with the strip lighting. It can be held tighter against the back and is easier to control. It also allows more leds to be placed inside, and the result can be dimmedto taste.
Some folks have reported success at eliminating the stripe effect by placing a frosted layer behind the frame, but that is at the cost of some brightness.