Making the cheiroballistra trigger is very easy. Here we forge it from flat steel ~3mm thick and 10mm wide. It can also made as easily by bending it in a vise.
The process step-by-step Edit
First forge the "hump" at a suitable point:
Next cut the handle part to appropriate length:
If you want, you can now forge the handle part from the sides so that it tapers to a rectangular cross-section. Then cut the trigger near the other side of the hump and drill a hole for the axle. The results should look something like this: