Step 1: Starting the Build
Step 2: Cutting Parts
From the CAD models, I was able to cut out the exact frame so that everything slides together and locks. I was also able to cut the top out of very thin ABS plastic sheet, which will give the 'rounded square' look once the LEDs are installed and lit up.
The reason for using black plastic is to try to keep this project as light as possible, while at the same time not allowing any light to go between boxes.