I was carrying my 97 the other day and wanted to bring a spare mag along, but didn't have any non-competition holders. On a whim I took my Victorinox SwissTool out of it's pouch and voila...fits perfectly!
Basically I just cut the flap off, leaving enough for a body shield. These can be had for about $9 shipped. There's enough retention so it doesn't slip out, but not so much it can't be withdrawn easily in a reload. I might actually use this for our next IDPA match.