I've read all the comments about problems people have had, and I think I might be able to help those who make this in the future
The cog is NOT symmetrical: the top of the teeth bend slightly. I tried it both ways, and found that I got very reliable triggering when the teeth slant forwards (i.e., makes it easier for the rubber band to slide off). When I had it the other way, I got lots of multiple triggers when using the single-shot trIgger.
The nuts will not fit, not matter what. On two of the holes I tried enlarging the hole with an Xacto knife. Big mistake. The nuts eventually ended up down in the infill area. It still worked, but doesn't look professional. On the other two holes I put the nut in place, lightly heated the nut with a soldering iron, and then pushed it down into the plastic (I used PLA for my print). That worked great and is definitely the way to go.
The trigger pin is too large for the trigger hole, but you most definitely DO want a snug fit. I spent 2-3 minutes sanding the pin to get it to just barely go in. I very briefly started to use a rat tail file on the trigger hole, but stopped almost immediately after I realized there is very little plastic around that hole, and I was going to make it weak. Sanding the fat part of the trigger pin is definitely the way to go.
- I had a huge amount of trouble printing the two body halves on my Ender-3. The problem is that, because the holes on both halves are recessed, and because you HAVE to enable supports in order to print the trigger guard, the printer has to also create supports for these tiny holes. Because there is so little plastic put down on these tiny support islands, getting the initial support layer for those sub-millimeter support to stick was next to impossible. It took about half a dozen attempts, but I finally found some Cura settings that let me make it work. The key settings were:
Initial layer height: 2x layer height (I printed 0.16 mm layer height, so the initial height was 0.32 mm).
Print thin walls: Checked (probably not needed, but I figured that these supports were so small, the slicer might skip them)
Retraction distance: 3mm (default is 5mm)
Retraction speeds (all three of them): 25 (default is 40)
Retraction extra prime amount: 0.064 (default is 0). I think this was the key setting. It causes over-extrusion for a moment after a travel
Minimum extrusion distance window: 3mm (default is 5mm) Cura help says to set this equal to Retraction Distance.
Support speed 60 mm/s (default is 120). This was to give the Ender more time to extrude when doing the little supports.
Initial layer speed: 20 mm/s (default is 30).
Initial layer travel speed: 40mm/s (default is 60)
Skirt/brim speed: 20mm/s (default is 30). I think this got change automatically. It doesn't really matter
Regular fan speed at height: 0.32 (default is 0.2). This is designed to keep those tiny islands sticking to the bed
Generate support: Checked (mandatory)