MakerBot Print is our newest print-prepration software, which supports native CAD files and STL assemblies,
allows you to interact with all your printers via the Cloud, and many other exciting new features.
I ended up using two springs pulled from a pen, with little metal nails slid into them to keep them moving the right direction. I had to drill an extra hole into the inside of the trigger.
It works reasonably well, but does not feel great, the whole trigger moves and grinds a lot despite a shitload of filing and sanding. The single hole on the bottom is placed in a bad location, it pushes the trigger at an odd angle so it doesn't want to return easily.
If you could find a long, but narrow flat spring, like the follower spring from a rifle magazine, that would be ideal because it would push evenly against the whole back of the trigger.