I'm ready to buy a CQB gun and I've decided I want a small AEG with an EG 1000. No ARs, cause I want something different. I want to mount a PEQ battery box on it for a large battery, and I want a box-mag for it.
the sig 552 comes with a rail on the side of the foregrip so you can mount the peq box there. i'm not sure if there is a box mag for the sig 552.
I have a G3 SAS. You can mount the G36 side rails to it (same size). Then I used the big CA peq box with 8 1/2 sub C cells mounted on the right side of the gun(think there's a pic in my gun thread). Not sure if the box mag will fit it, but I think it should.
Cool idea about building a custom battery to fit in the box mag. I don't know why someone doesn't make a box mag that has a built-in battery compartment (aside from the P90)