I'm running AR mags, and got it milled for AR mags. Shouldn't be much different for G36 mags, and I don't think it would be excessively wobbly. You could ask him, I'm sure he'll know.
USGI mags lock up pretty good with a tiny bit of play, and they drop free nicely. I typically use Israeli Orlite AR mags in my SL8, because they are pre-ban, black plastic, and match the look of the SL8 nicely. They even have guide holes for drilling out if you want to make the rounds visible ala the G36 mags.
The steel reinforcment is CLUTCH. The work needs to be seen to be believed. He got my gun back to me in record time. All in all, a great experience with Tbostic and as soon as I figure out what I want to do with the rest of the rifle, it may well go back to him a second time for a barrel chop and thread job.