Sorry to revive this old thread, but I finally got around to finding something that fit my lack of a machine shop and desire for a fun project, and thought I would impart this knowledge on you fine folks.

Enter the 80% polymer lower. Specifically from ep lowers.

It is your run of the mill polymer lower, but there are no pin/fire-selector holes, the handle attachment hole should be tapped (can be done carefully with the screw), the trigger hole needs to be widened out, and a big chunk of resin (and support tabs) need to be removed from the fire control pocket.

However, all this can be done with a dremel, a hand drill, a file, careful approach, and a good amount of patience youtube can tell you how. Disregard videos where the person finishing it used an actual mill, that is way too aggressive for the materials it seems, and a dremel or drill press seems much more ideally suited.

There are pitfalls though, using a handheld tool it is extremely easy to let it walk all over the lower and not just the parts you want to remove. And of course if you're not sold on the idea of a polymer lower to begin with, that is a problem too.

Having looked at this particular product in person, it seems like it would work just fine, but they should maybe have reinforced the sides of the fire control pocket and the magwell a little better (there is a tiny amount of flex, but that may not be a bad thing), it seems much like a straight up copy of a metal lower.

There are now several similar products on the market, some have one-time plastic jigs and requires a drill press to finish.