Something small enough to fit in your pocket should maintain a more constant temperature. It's not the temp but the changes, namely anytime the temp increases, be it from room to hot or from cold to room. A few things can help, when I leave clearos in the car I put them tip down so most of the juice can't escape. The other is normal anti-leak cleaning techniques. For example, EVODs and Protanks have a little tube out of the top of the coil unit that should be wiped off every fill, to avoid multiple tanks worth of normal coil spitting trying to take the space that nicely accomodates a tank worth. If pulling the coil and base out to fill also releases any juice out the mouthpiece, clean that as well. If you don't do this you will eventually get leaking and spitting even at constant temperature.
I do recommend the EVOD clearo for many flavors and the Mini Protank 2 for juices needing glass. The flavor is great if configured properly (voltage less than 3.7/about 5.2 watts, higher resistance coils or they will burn on longer drags). A decent 650 maH battery should last all day (if not, get 2). The only decent starter kit for them I am aware of is the MyFreedomSmokes EVOD which costs $8 and their current version will probably need the coil replaced immediately with a Kanger 2.2 ohm. Every other starter kit I've seen is fixed voltage, unregulated battery, or can't be adjusted down low enough. These units are not any more or less leakproof but they do the best combo of flavor, size and weight. Avoid the EVOD2, Protank 3, most or all Aspire BDC/replacement BVC stuff as they use twice the power, 3 times the juice and often deliver less flavor in the bargain.
Another option would be a dripper. They leak immediately and profusely when not held upright unless almost empty or a lot of wick is used, but they are completely unaffected by heat except of course the battery probably shouldn't be worked too hard when it's 112 degrees.
