If you wear tight pants, maybe the back pocket. Looser pants, use the front pocket with something like a Desantis Mag-Packer. If pockets are not an option, try belt carry with something other a mag pouch: cell phone holster, knife or Leatherman pouch. For your gun, I imagine a small cell phone holster would work well for that extra mag.
Your idea of adding a pouch inside the waistline sounds good. In fact, if you start with some sort of pocket mag holder (like the Desantis, or an even thinner leather based one) you may be able to find a way to attach it to multiple pairs of pants IWB so you aren't modifying all your pants.