Recently, (this week) picked up a pair of mag carriers. Amazon, IDOGEAR was the seller, they are a combination of a rubber wrap sleeve that retains the mag, a plastic skeleton around the sleeve that gives it rigidity followed by a bungie cord that can be tightened or not to custom fit for your mag. The one I have is designed for any double stack 9mm mags. So far I note really good retention, almost too good, I may apply a bit of armor all to give things a bit of lubrication. They came with an option of attachments, either molle type or a clip on to a belt type. I run a belt when at the range so belt clip on for me. The top is open, no retention on top, just the sticky rubber and tension of the wrapped bungie.
There is a name for these, i think they had been referred to as taco holders, unsure of that. I have seen them used in videos I watch for training drills.
PS, forgot to mention, I have this same style of mag carriers for my AR, I greatly prefer these to a fabric nylon that has no skeleton holding it open and top side retainer bungie that can get in the way.