The
Rider Robust is a nice option for the original poster. Full featured variable voltage tube mod made from lightweight aluminum. Be aware though that if you choose to use anything above 4.5 volts for a length of time that the small 14500 battery will not last long at all. As long as you know what your are getting, it could be a good option for you. Here are two reviews:
A PBusardo Review - L Rider Robust and EZ Turbo Winner - YouTube
RIDING WITH THE L-RIDER ROBUST - YouTube

I own all three of the below mods and can recommend them as lightweight with a small form factor and easy to use.
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The
Segelei V8 is a full-featured variable voltage box mod in the "gripper" form factor. Made from impact resistant plastic with a ruberized finish. This is about the size of a large smart phone, but thicker. I have one of these and it is a fine mod. I got the optional leather lanyard which is convenient for out-and-about use. This has a built-in atomizer and voltage meter. It takes two 18350 high drain IMR batteries to work, which I have found to last me all day. $30 for just the mod; additional $9 for the leather lanyard at Discount Vapors.
Sigelei V8 Incremental VV Body - Black Rubberized
Sigelei V8 Leather Pouch
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The
VMod from Vapage is a mechanical bottom feeder mod that is small, lightweight, and pocketable. It comes in two sizes: a VMod 2.0 (14500 battery) and a larger VMod-XL (18650 battery). My VMod-XL is the first pictured above. $85 for either model.
http://www.youtube.com/watch?v=HzN9dQWsfR8
The VMod 2.0 has a 4 ml juice tank that fits inside the mod; the VMod-XL has a 9 ml juice tank. To work, you turn the mod upside down and squeeze the soft plastic juice bottle to feed an amount of juice into the drip well at the top of the mod. Then turn it upright, do a primer puff to feed juice into the bottom of the atomizer, and then vape away. After a few vapes, you will need to repeat the process to continue vaping. There is absolutely no leakage of juice. The 18650 battery on mine lasts two days of use, and the 9 ml juice tank lasts me three days before requiring a refill.
VMOD 2.0 and VMOD XL
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The
Smoktech VV Gripper is a full-featured variable voltage box mod in the gripper format. Made of impact resistant plastic with a rubberized finish. Slightly smaller than the above Segelei V8, but same pluses of being lightweight and pocketable. Built-in voltage and atomizer meters. Uses a single 18650 IMR battery for all day use. $50 at Mad Vapes.
Variable Volt Gripper
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An excellent source for batteries and chargers is RTD Vapor --
http://www.rtdvapor.com/online-store/batteries