Most fanny packs involve zippers, some use velcro and some are open on top. If you want to go that route, Kahr makes a decent, but expensive one. I think that link for the 'Belly Band' looks pretty good and hey--they even make compression underwear and T shirts, with built in holsters!
Overall, I don't think fanny pack 'system' is a real good idea and they usually hang a bit loose. I used to do some brisk walking, even some light jogging, in gym shorts with a handgun with no real safety, other than what you typically get in a M&P or Glock's trigger. I'd just tuck it into my waistband, in the small of my back. That usually worked fine, except for one time, when it came undone and ended up down in the groin of my underwear and there I was, jogging along with my loaded pistol 'down there'. Very stupid! Potentially worse than when I went horseback riding and at full gallop, the saddle totally slipped and I was under the horse! That was quite a rush
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Anyway--I find that a belt slide, leather concealment holster, worn inside my short's elastic works for me, even w/o a belt. I can jog--my shorts are usually a bit snug and if my pistol's going to go way down, I'll be aware, as I'd be inadvertently 'dropping my shorts' at the same time and risking arrest. The leather sticks to my body pretty good and I can also wear it more formally, on the outside of my belt with different sizes of pistols as well.
Ebay (fleabay?) has bunches of holsters of all types, including belt slide type and others, including some M&P specific models.
