The point subject to the highest stress is the fillet area just forward and below the buffer tube ring. I have a finite element analysis of it somewhere, I may be able to dig up if you are interested in such things. I was at the design-to-part show two years back and saw a sample part that was...
It's hard to go really wrong with a lower (other than a DIY plastic one), since the job of the lower is basically just to hold the trigger group and other related parts in proximity to each other. The "mil spec" parts tolerance are in some cases as liberal as .020" deviation on some dimensions...
