I tested the idea of a single trigger without a nut to keep things simple, but I realized the pivot of the trigger was below where the trigger latched, meaning it could slip off on its own potentially. To eliminate this I went back to the idea of using a nut. The nut has a built-in lever so that I can easily reset it after firing. Below is the CAD process.
Step 1: Here is the side profile sketch I came up with after tinkering with it for a little bit.
Step 2: Extrude the trigger (blue) and nut (red). Also extruded cylinders for the pivot points.
Step 3: Cut material away from the handle so the trigger mechanism has room to operate
Step 4: Add a built-in return spring to the trigger, hopefully this won't just snap like a twig...
Step 5: Cut holes for the pins to rotate in
Step 6: Add fillets
Step 7: In order to be able to assemble the trigger parts, I am going to split away two little pieces of the handle that will act as retaining plates. You can see the sketch and the finished split parts in the two pics below.
Step 8: Add holes for screws, I used No. 2-56 screws because that's what I had lying around. (explode view)
Here is the finished trigger action. After a shot, I can reset the trigger by rotating the red lever. We'll see how it 3d prints!
Some other shots showing all the parts: