I built my blaster EE3(i know its not the same as the KYD-21) around a .44 magnum die cast cap gun just to get a weapon shape, (particularly the handle/trigger) and rigidity . it was a good starting point..
I unfortunately do not have access to a 3d printer. Unless it was pepakura you were speaking of. I am attempting to construct one, at least the basic build of it, out of wood. shouldn't be too hard im guessing using different dowels. Going to change up the muzzle brake and add a few mods such as a partial trigger guard and sights. Assuming I am successful on this attempt.