Apex's website show they have used G3 mags for $7 and $9. That may make it worth considering if I was in the market for an .308 AR style rifle.

The G3 mag has an extra layer of metal at the top so it is wider and longer that most .308 mags. The mag well would have to accommodate that. I doubt the mag well wall thickness would allow widening a standard lower. The G3 mag also has insertion stops closer to the top. The well would have to be shortened or groves cut to accommodate them, again I don't think a standard lower would handle that. The latch is different also.

So basically I don't think it would be possible to modify an existing lower.

If you do a 'net search using your post title you will get some hits. One of the hits I got was a 2009 report that CMMG was taking deposits for some lowers. If I was building a .308 rifle back then I would considered it if I could find one.

Hope this helps.