This kick-ass free-to-use pattern (.pdf) for making your own screw-together AR-15 lower receiver from sheets of aluminum of varying thickness has been making the rounds again what with the recent passage of anti 3D-printed gun laws. The original website is sadly gone.
All you need to do is cut them out and screw them all together, then drill and tap a couple of holes. Guess which one can be shot more?
Might have some problems with some aftermarket mags, though. Might want to consider trimming that magwell a bit should you try this yourself.