mylk [mech] Box Mod
Duel 18650 Mechanical Mod with "Fat Daddy Vapes" 510 connector, chemical etched skulls design and (neodymium) magnetic battery cover closure.
Made this for my brother as a Christmas present using all the techniques I have learned making guitar pedals and mini amps. Made from a standard Hammond 1509b guitar pedal enclosure etched with a combination of Ferric Chloride, copper and Citric acid.
"How to build an e-cig Box Mod"
Stuff to Know 
- Batteries in Parallel = 3.7 Volts (Double mah)
- Batteries in Series = 7.4 Volts (Double Voltage)
- How much power am I using ? - http://vapecalc.com/
- Pic showing Correct way to wire batteries in parallel (Full article here)
"How to build a mod" Youtube playlist
Variable Voltage 
LM2596 [Step down buck converter]
Can only reduce and regulate voltage, Not step up. Use Batteries in series (7.4V) and reduce as needed
LM2596 Datasheet - Here
LM2596 Mod Builders Forum Board - Here
Notes on Trimmer value for best Voltage range - Here
Very easy VV mod - Forum post for small 2x 18350 LM2596 Box mod
LM2596 - VV Mod Schematics from (http://www.e-cigarette-forum.com)
Only reads volts at the atomiser, no reading for battery voltage. To be able to read both battery and Atomiser voltage use schematic below ................
Awesome site with great MOD building tutorials and parts kits