There are many top quality brands & designs for double magazine cases, for duty, range, open or concealed use.
5.11 Tactical, Blackhawk, Uncle Mikes, Blade-tech, COM Holsters, Galco, Alessi, Greg Kramer, Mitch Rosen, Bianchi-intl, Tuff Products are all worth a look.
I like to use a covered type magazine holder to prevent dirt, dust, grit, lint, water, etc from damaging the pistol mag(s). It's a slower draw but Id take that trade-off to prevent any problems with spare magazines.
I've also had open top holders cause the mags to jab my side or scratch items(chairs, car interiors, etc).
Clyde