I recently came across a commercial rubber band gun, and decided to make my own. This is the (still untested) result.
Cut from acrylic, wood or what ever material you see fit. Assemble with four M3 bolts and load up with rubber bands (this is a 4-shooter, but depending on tolerances, it might hold more with a little tweaking!).
The thin blue lines are just vector engraved lines, to mask the area holding the trigger (you can omit these if you don't use transparent acrylic).
This file will result in 3 layers to hide the trigger inside the two outer layers, but you could go light on materials and just use a washer on the end of the M3 to hold the trigger in place...
I'm working on another version with a spoke wheel release system, but that's for later enjoyment!