For stick-battery/carto combinations, or twist sized batteries with a carto, I use a shirt or jacket inside pocket. I also have an old pen holster which is perfect for holding 3 stick batteries.
I didn't like the looks or size of the case Innokin made for their box style devices. For my MVP, I use an old Radio Shack cell phone case with a strong belt clip. It's almost exactly the same size as the MVP. The carto or mini ProTank sticking out of the top looks like some sort of geeky retro antenna to most people -- I get more questions each day about the "cell phone" on my belt than I'm asked in a month when I actually pull out the MVP and use it.
If I'm going someplace where I expect to be away from home for more than a day, and will need several batteries, a charger, etc., I use an old eyeglass hard case or two.
I like the size and lightness of cartomizers for when I'm out shopping or going to the park. Typically, I'll prefill two or three of them, replace the rubber condoms on each end, and carry them in a small, pocketable SD card case.