Here's my type up from where I posted it in ASGIF's mini-mod thread. Slightly different.
I have another mod for an AUG to fix the broken/sheared/cracked "motor plate" that you might find on an AUG from the AS Megastore bone yard. This "motor plate" as they call it just holds the gearbox tight with the receiver. This can also be done with an AUG that doesn't have a broken motorplate to free up some battery space.
1)Acquire two allen screws long enough go about half way through the receiver at the point right behind the gearbox. Leave a few millimeters space between the receiver and screws. You'll also need a bolt coupler the same inner diameter as the allen screws.
2) Measure the distance inside the receiver from the end to the part of the gearbox you want to secure it at. I put it in the middle part below where the spring guide would be.
3) Mark on the outside of the receiver where the bolt should go.
4)With a drill bit slightly smaller in diameter than the allen screws you chose, drill trough both sides of the stock at the same time to ensure the holes line up. try your best to keep it as horizontal as possible.
5) With the allen screws, screw them into the holes partially. The holes shouldn't be so much smaller than the screw that is a job on its own to screw them in, but small enough the fight tight. Attach the coupler between the screws. You'll probably need a wrench to keep the coupler from spinning while tightening the screws.
If it doesn't keep the gearbox snug in place, you put the hole too far back. Have no fear, for electrical tape is here! If that occurs, just take the screws out and wrap the coupler in enough tape so it pushes the gearbox all the way forward. After that, you might need to use a bigger wrench to tighten it, or cut enough tape away from the center of the coupler so the wrench fits while retaining tape on the sides to push the gearbox. I'm not sure how well the tape method would work with the coupler since I got it pretty much perfect the first time.
It frees up a fair amount of battery space. I have no reference to how much since I never had the plate in to begin with, but I've talked with other people that have done this, and they say it frees up a good amount.
It looks very clean on the external side.