There is, or at least there used to be, an outfit that made ARs in .45 that used Grease Gun mags. Can't for the life of me remember the name of it, but I'm 99% sure it started with "B" "B-something Bros," I think it was.
Quarter Circle 10 makes lowers that use Glock mags, and their "Large Frame" lower will work with either .45 or 10mm mags. That would be a "build-it-yourself" proposition, I'm afraid, and probably outside your budget too.