I've often carried a spare mag for my Hi Power in a belt-mounted leather sheath/case made for a folding knife (mine is a Buck, I think). I just stuff a little paper in the bottom of the sheath to get the mag to ride at the height I want, and I'm good to go. The sheath/case has a snap flap and looks for all the world like a simple knife and/or Leatherman style tool case.
