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.
The author marked this model as their own original creation.