Very creative. Maybe you could try something like Drac's miniclip design.
I considered chopping one of my 6-6rnd mags down but couldn't bear it. I like the idea and would suggest it, but they can get pricey, especially considering I'm broke.
I had a think though. a mag made of framework and powered by elastics. i made some bends on steel round stock and got this:
needs bending and cutting here. just the best pic i got. two elastics. one on the frame at the rear, another on the inside of the barrel.
fully bent, cut an inch from the last bend.
it broke while i was spreading it. thats how open it needs to be. ill weld it likely monday and update.