I'm going to dodge the 6V specification and, instead, recommend a
variable voltage
device used with a cartomizer-surrounding
tank. If 3.7V doesn't cut it for you, why limit yourself to a high fixed voltage (the the relatively limited number of attys and cartos with appropriate resistance to use on it)?
In ascending order of price, the
Buzz Pro,
ProVari, and
Darwin seem to be the most popular VV tube mods today. There are less expensive plastic box mods, such as SmokTek's
SmokBox VariCool (recently
reviewed by CyberWolf), Ken's Box Mods'
Big Brother and
Little Sister Vari-Cools, and MadVapes'
Variable Voltage Box Mod (not varicool). [According to CyberWolf, the varicool technology "allows the unit to run cooler and longer while providing optimal performance."]
The cartomizer-surrounding tanks have become extremely popular as modest-to-moderately-priced auto feeding systems. [No bottle squeezing required.] I acquired a
VPX Cartomator Cartomizer Stainless Steel Tank at VaperCon last week and am loving it. Mine is 3ml capacity, but there also is a 5ml version (not currently available).
Phil Busardo has praised his less expensive Delrin
J-Tank (capacities of 3ml to 5.5ml). Similar highly regarded tanks are the 3ml Delrin
DCT MAP Tank and the Delrin or Stainless Steel, 3ml to 8.5ml
Vape-O-Matic Liquinators. And there probably are others I am not yet familiar with. All of these use hole punched cartomizers (dual coil or Boge), available pre-punched or punched by user.