Just a tidbit of info in case you didn’t know: dual 21700 batteries with be on par with triple 18650’s, as far as battery life is concerned, and the mod will be a bit smaller. Don’t know if that matters to you but I thought it was worth mentioning.
I've got a IPV 4 or 5 (can't remember which).that's a dualie (excellent shape), and a really beefy single battery one from Project Sub Ohm. Something "Mini", which is hilarious because it's about the size of a VW Bug.
If you want a "beefy" mod running dual 18650's, look at the SMOK Mag P3. It's a handful. Doesn't come in all black, though. I've had mine for several months and still lovin' it. Not a good travelling mod, given its size and weight. And it goes though power like there's no tomorrow.