That would be way too thick like Spud Spudoni said. No human would have that large enough hands to grip over a magazine and a plunger. Even if it were possible, there would be no way of connecting the priming handle to the plunger head. You'd have to make it prime from the bottom or something, and that'd defeat the purpose of this homemade.
12 inches for a homemade pistol meant for NIC level gameplay isn't very large. Nitefinders and Firestrikes with speedloaders are around the same length.
Two plungers mean that you have to prime twice to fire one shot. Also, with the two priming handles on opposite sides of the magwell, it'll be awkward to insert a mag.
A rectangular plunger system would be incredibly hard to manufacture. You'd somehow need to flatten PVC to get the plunger tube. Then you'd need to somehow source a rectangular washer or o-ring to get a seal. Finally, finding a spring that would fit inside a plunger like that and provide adequate power is next to impossible. Just look at the Pocketstrike's internals.
I personally think that the current design is great and Captain Slug is working hard to make it a reality.
I know it would be thick, It was just an idea I was playing around with. But for the priming it could be linked in to the slide with fishing line or steel chain thing that the project currently uses.