I designed this lightweight ecig battery in Fusion 360. It's ergonomic curves fit the hand nicely, and it doesn't feel bulky in your pocket like other mods. The case features a slide and lock mechanism with incorporated clips for all components. For safety, there is a flush-mount button, on/off switch, and two 15 amp resettable fuses. The Keystone 1049 battery holder has pegs that pop and lock into the case. The fuses are small enough to mount flush with the holder (put a slice of kapton behind the fuses after soldering). Mount the IRLB3034PBF mosfett to the holder as shown with a small screw. I used copper tape from adafruit as the negative lead, but there is enough clearance between the holder and the case for a wire. I got the switch and 12mm button from adafuit also. This design uses a salvaged 510 connection from an old 'Ego' style battery, so you can use ego atomizers with this mod. Be sure to use heat shrink tubing on all connections and kapton tape between all components. I had to trim/bend the leads to get everything to fit properly. Use E6000 to glue the switch, button, and 510 connector. You may have to work the slide and lock mechanism back and forth to get smooth operation.

Tips:
-Prints without support material!
-I had to bend the ends of all the battery connectors inward because they were catching on the battery wrapper. I used needle nose pliers.
-Bend the leads of the mosfet up and over to clear the button.
-I found the wiring and assembly guide found here to be helpful http://findmyvapes.com/how-to-build-unregulated-dual-18650-box-mod-with-mosfet/
-The old ego batteries are common and you might know someone that has a broken one in a drawer where you can get the connector.
-Adafruit components product numbers: switch 805; button 1119; copper 1128

Do this at your own risk only if you are comfortable with electronics.