A few people have done the same mod, they use the internals from a sound-activated force-fed hopper and can get about 20 BPS and 90 rounds into a mag. Not alot, since the donor hopper generally carries 200 rounds, but it beats the 20 rounds my current guns hold, and I can build one for about the cost of one and a half mags.
I tried to scratch-build a box, but couldn't come up with one I was happy with. They mostly looked like fail, since I don't have the tools to properly cut lexan or other materials that would work well.
But thanks for clearing that up for me.