I've been wanting to do something like that for a while, but never had the time. Was considering using either an SKS or an AR for the basis. SKS might be easier because it's top loading. Potato Digger should be easy, since the body of it is just a rectangular box. But this morning I had another thought. What about a completely hollow mock up, with something like a Ruger 22 auto pistol , or some other fixed barrel auto pistol, clamped into the rear where the pistol grip would be anyway. Just firing through the body of the machine gun, or even clamped into the barrel tube and firing through it? All the normal controls would be exposed, no complicated trigger linkages needed. But....I wonder if it might draw disapproval from the BATF, converting a .22 pistol into a crew served weapon?