insulin

Something small like this? Tabletop ice machine.
The one I have says it draws 2 amps at 115v or 230 watts (2A x 115v= 230watts). If you have an inverter hooked to your car battery or a booster pack with built in inverter you could easily make enough ice to keep small stuff cold.
You would need to have your car running for the 10 or 20 minutes it would take to make enough ice to last a day in a decent cooler.
240 watts = 12v x 20 amps

try this no power clay pots sand and water http://www.modernbushman.com/2012/03/29/zeer-pot-electricity-free-refrigerator-emergency-refrigeration/

If you need to keep life saving meds cool...be sure to try your method first!
Sure, the zeer pot in the link DOES work, but only to a degree and is weather dependant.
Scroll to near the bottom of the article and you will notice that the temperature inside the pot only cooled to 69F...certainly not cool enough for insulin.
IMHO, if you are looking for a 30 day method...the small ice machine is the way to go. Even a small 1500W genny that you can get for $200 or so will do the trick.
If you only run it for an hour 2 - 3 times a day, you would be surprised how long 20l of gas will last.
While you have it running, charge up your cell phone, rechargeable flashlights and radios, portable DVD player, AA or AAA batteries, etc.

there is another that goes in ground same theory clay pots sand covered ,