Well, concerning box mods:
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The Smoktech Gripper is only $12.50 and very small, excellent if stealth vaping is ever an issue.
SMOKtech 18350 GRIPPER Mechanical - Black Rubberized [gv-SMOKGripper18350-BLACK] - $12.50 : GotVapes.com, E-cigarette Supplies - Atomizers Cartomizers Mods Juice and more
It uses an 18350 battery
Batteries : AW IMR 18350 3.7V 700mAh rated for about 7-8 hours use, but extra batteries can be carried to last out a day. Any common XL length cartomizer can fit into the mod, so that only a drip tip extends from it. An extender can also be used to allow other juice carrying devices to be used on it like a clearomizer, cartotank or clearotank (vivi nova).
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I just bought an Ovale/Segelei V8 Incremental VV
Segelei V8 Incremental VV Body - Black Rubberized
this past weekend to replace a Smoktech variable voltage Gripper that I was using for out-and-about use that only lasted 7 weeks.
Comfort-gripped, rubberized finish. Variable voltage. LED display. Reads atomizer resistance and battery voltage. Has easy on-off, voltage up-down buttons. Light, easy to fit in front shirt or a coat pocket. A leather pouch/lanyard is available on the site. Only $29 w/o batteries. Uses 2 - 18350 imr nipple-top batteries which together should last at least 14 hours per charge.
Batteries : AW IMR 18350 3.7V 700mAh.
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The above are all made of plastic, hense being less expensive than the metal tube mods. There are other box mods that are made of metal such as the Inokin MVP for around $80 which uses a internal nonreplaceable rechargeable battery.