some small ac units can be run from a 1000 watt honda inverter generator. since with such a small space the ac will likely cycle off quite a bit you wont be burning that much fuel.
the hard part is figuring a way to mount it safe and effective in such a small rig.
doubt there is enough roof to mount sufficient solar to run an ac during the day and probably not enough space or weight capacity to carry enough batteries to run the ac through the night.
while, where there is a will (and deep pockets) there is a way. i think you are in a tough spot. it is going to take a very complicated and creative solution to get air conditioning all night in a focus.
if you have to have a small efficient car, i think trading in for an older prius might be your best bang for the buck. they, being hybrids are designed to automatically start the engine to recharge the traction battery when it gets low. and the air conditioning/ climate controll will run when the engine is off. it is built with this in mind and the charging system is built to do the job. the computer monitors it all. i have seen some blogs where guys in like vegas only use a couple dollars a night in fuel to keep the car 72 degrees.
if your in the tough spot of needing to be in hot/humid areas for personal reasons with out the funds to rent something airconditioned. or have a friend you can park and plug in at night. i dont think you will be able to afford the cost, up front and on going of trying to add an airconditioner to run at night. if you are willing and able to live in a small car. i would serriously consider shopping for a prius and ditching the focus. i think that is the only realistic shot at being comfortable. if not, drive (move) somewhere cooler and talk on the phone or have them come visit you.
only you can weigh the scarifices you will have to make to stay there