You don't need mega wattage. you said that you're an 'MTL' vaper, so you CAN and SHOULD be vaping at lower wattages. That means you can stay with small (stealth) mods and still enjoy long battery life. It's only those wishing to produce clouds (non-stealth) that typically need higher wattage devices.
With that said, there are in fact a LOT of tiny little mods (both internal battery and 18350 based sticks) that should suit you just fine. If you build your own (RDA/RTA) atomizers, then I would suggest the Goblin Nano RTA's. They make for a really stealthy tank, and are perfect for MTL vapers.
Here's my two favorite stealth setups (in front of a deck of cards for size comparison).
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COV Mini Volt with Goblin Nano in black, and a SMPL 18350 with Goblin Nano in stainless. Between the two, I can stealth vape (and flavor swap back and forth) all day long... no problems with power consumption.
Finally, I'm not sure if you mix your own juice. If not, you SHOULD. A key part of stealth vaping that few have discussed is cloud production. You want minimal or no clouds at all. Blowing clouds in public places is NOT stealthy, and the jerks who do it, are the reason why vaping gets banned.
While at home, I vape 50/50 VG/PG at between 3 and 6mg. For my stealth mods, I mix at 100% PG (makes almost no clouds), and I bump up to about 9mg nicotine. This means I can take smaller hits, hold my breath for a few seconds, and exhale NOTHING. No tell-tale clouds at all.
I frequently vape in stores, restaurants, theaters, malls and in the homes of friends and family. As long as there's no clouds... nobody cares. They don't see the mod, and they don't see clouds, so there's really nothing to whine about (except maybe a slight Skittles scent).