Then I designed some supports for the base, in order for it to be less flexible, and also so that the catapult could rest flat on the surface despite the rubber bands going under it:
The last thing I wanted to add was some sort of triggering mechanism, and so I designed this, taking advantage of one of the rubber bands used for shooting to also hold it into place.
Once held in tension, just press down the little lever on the back and the catapult will shoot!
I printed with 0.2m layer height, in PLA.
Once printed, just slide the trigger into the back part of the catapult's base, and hook one of the rubber bands to the protruding edge at the very end of the trigger's base.
To shoot, pull the catapult's spoon back until it locks into the trigger, load the catapult and pull the trigger's lever back until it releases the catapult.