You don't mention what firearm you are searching for magazine holders or whether you prefer leather or Kydex. I can whole heartedly recommend these folks for their Kydex fitted clip on holders. Very low profile for concealment, easy on/off yet I have never experienced a magazine coming loose or the holder coming loose, even while taking part in some very active training. I have their pouches for over a dozen different firearms. KYTEX Shooting Gear – Shooting Accessories for Competition, Carry & Duty
I have a clip-on holder for a single Glock mag, it’s ok… but then I got a horizontal mag pouch, that sits along my belt rather than vertically. SO much better, for comfort and pocket access!
Comp Tach kydex holder works great. It has a forward facing lip on the clip that assists with sliding onto belt and also grabbing with finger to pull clip away from belt for easy removal. Their holsters are very good also.
Regarding security of mag pouches and holsters, I have several of each and never a problem if they are properly placed on the belt. In fact, I sometimes have trouble getting them off when the Carhartts need washed. As always, a good gun belt is a necessity.