May be a little much but beez combat makes a fancy overpriced phone pouch. It does have two straps for molle but you could use just one. Even comes in multicam black or straight black to match the bag.
The Phone pouch holds one mobile phone. The flap does 3 things; acts as the primary way to secure the phone into the pouch, helps with screen protection by covering the phone, and also provides a laser cut pull tab to facilitate access to the phone. This pouch is a new piece of military kit that...
How much retention do you need? I'd try this for compression fit if you didn't want a flap. I have one of the horizontal models mounted on my belt and it'll hold a loaded mag like a champ.
The Ten-Speed Single M4 Mag Pouch holds one M4 magazine (5.56×45mm NATO rifle cartridge) - compatible multiuse with flash bangs, bandages, and other similar sized items.
I ultimately ordered one with a velcro flap. Once I get it I'll put together my sling bag and show you guy what I did do you can point out the stuff I didn't consider properly or forgot entirely.