Rather than spend any money on MOLLE crap or the like, I simply use a surplus canvas gas mask sack. It has a shoulder strap on it, a couple of small compartments inside, and can easily hold six (if I wanted) AR-15 magazines, a couple boxes of ammo, spare Glock mags, a flashlight, energy bar, chem light, harmonica, whatever I would need if I had to take off out of the car for a while and leave my bigger gear bag behind.
Being facetious. Mine in fact has three AR mags, two pistol mags, a bottle of water, and a spare radio battery, plus a chemlite or two and a granola bar.