So after a few weeks of playing with the idea I've finally gotten all my parts together. I took to dpms mags, had them cut and welded together at 8 3/4" from long point to long point. (l measured a 30 Brit fal mag that I had laying around.) I then bought a pro mag 30 round 308 mag, took the spring out of it... Presto 30 round frankinmag! I've hand cycled 28 rounds out through my M as fast as I could with no issues! Can't wait to try it out tomorrow at the range! Total investment (not including the POS X25 mag I bought that inspired this project) is $110, I payed $29 for each of the mags, $15 for the promag 30, also pretty sure I can do the next one for less, by better sourcing the mag bodies..
I've always thought the magpuls are ugly as hell, but each his own.