IF you want both easily rebuildable AND stock replacement heads, your options become limited.
As Highwayman said, the Subtank mini or the Lemo 2 would be good choices.
But neither is exactly cheap nor small.
The GS's are nice for cheap and small and replacement heads, but not easily rebuildable.
If stock heads are not of primary importance, the options become much more numerous.
You mentioned the Erlk/Nano.
I LOVE my Erlk's above all else by miles and miles.... and miles...
You can get an authentic for $$, a Prince for $, or a 'stylized' (clone) for $5 from AngelCigs.
(get 2 for $10 - each gives me 20ml before rewicking = 40 ml = 4 days)
(I've got 1 authentic and 3 AngelCigs clones = all my many other tanks sit on the shelf ha!)
They are monster flavor, light, small, cheap, and hold 8ml. Watch a video for wicking, not hard, just specific.
A nano kit is provided at FT if you wish ($3). I find the small size difference vs. juice capacity a poor tradeoff though.
And if you get an Erlk clone and like it, can't hurt to support the developers and get an authentic for home use.
EK Style Rebuildable Tank Atomizer (8ml) White
$2.66 Replacement Chimney and Tank for Mini EK RTA - stainless steel + PC at FastTech - Worldwide Free Shipping
Thanks, capthook!
I have a 1:1 Erlk clone and LOVE it, even more than my KFL+v2 clone. I don't think I want to deal with a build gone bad while backpacking. The entire object of my quest is a good vape while spending a week or two on the trail. Inexpensive gear is desirable should disaster strike. If a head goes bad, I simply swap a new one in. I've been rebuilding Kanger heads for ten months without a problem. @Cheechako 's video makes the GS heads look no more difficult to rebuild. As for nano vs. regular Erlk; I seldom put more than 3ml in my Erlk at a time anyway. I'm told the flavor on the nano is even denser/warmer than the full-sized. At <$10, I guess the GS Air M/MS is a cheap experiment should I end up not liking it.