The slot is 1.47" wide, and accommodates proofgrade medium acrylic. (It will fit the edge-lit Neil Gaiman quote I uploaded a while back.)
(Note, this was designed before I learned about kerf, so it requires gluing.)
The small bits are to glue around the slots for the acrylic and the USB cable, to help hold them securely. The USB slot is tight and will take some wiggling to get the cable end stuck in.
Here’s what it looks like on the inside, after assembly:
The bottom is open for access to the LED, since it’s touch-operated.