I've messed around with a new cad/modelling software and just ended up with a pistol somehow…

Hardware needed: A bit of 4mm bungee and maybe some tape for the bungee where it interfaces with the lever to avoid too much flex

Assembly, put the trigger in the receiver, secure it with the triggerpin

then put the grip in place, slide the lever over and secure the three parts with the leverpin

then wrap some bungee through the holes and the acceleration slits and tie a knot in the gap under the muzzle.