I like the idea of finding a phone/ipod/whatever holder that will serve as a mag holder. I also like the idea of Leatherman/Gerber/etc. holders. I use one periodically. But in case you none of these options fit your needs, perhaps one of these links will help:
Accessories and Belts by Mil-Tech and offered by Gunny Sack
Scroll down and there is a double mag holder. Not sure what it would look like to others, as it's made to be worn on the outside of the belt.
Military Surplus - Concealment
Several double mag holders here, but most or all are OWB. Item 123, the Safariland Concealment Magazine Holder, Horizontal is a good item for one mag. It has a velcro strap to attach it to the belt so you don't have to thread it on. It's quick on and quick off. I have one and use it periodically. Unfortunately, I found it at another website and got to pay extra for it.
8002-313 - BianchiĀ® PatrolTekā„¢ Nylon Magazine Holder - All LE/Security Products - Bianchi PatrolTek Nylon - Nylon Duty Gear - Red Diamond Uniform & Police Supply, Inc. - Detail
This is another double, but again, it's OWB.
Raine Inc. - Ammo Pouches & Cases
More doubles, still OWB.
Milt Sparks Holsters
Click on Products, then Accessories. Model IPD is IWB, and only the straps will show. The D-4C looks like it is OWB.
IWB Single Magazine Pouch
This is a single IWB, tuckable. Again, the belt clip will show. Pricey.
Andrews Custom Leather
Scroll down to Concealed, Magazine Pouches, and check out the Pancake Mag Pouch. Says it's concealed, but looks like it would take a cover garment.
Gun holsters concealed by SmartCarry concealed holsters
There's always the SmartCarry. If you want to carry an arsenal, just get the security model with an extra full-length pouch.
I looked for a long time for exactly what you are looking for. I never found what I wanted, so I store one extra mag in my SmartCarry. If I want a second, I can use either the Safariland 123 (looks like a phone holder) or a Leatherman holder. I've got this on order
Deep Concealment shoulder holster,With the DeepConcealment you can conceal your gun with a plain shirt,For correct sizing please measure your chest's circumference about 4" below the armpit. which also has two mag holders opposite the holster. If you order this rig, go small. According to their charts, I wear a large, but it was WAY too big. I sent it back and have the medium on the way.
Good luck.