A little more Science (and why I chose the icebox)
Weasel is right on with the math, except that he understates the btu requirement by a considerable margin (for simplicity's sake) by only considering sensible heat transfer in his example. In reality, the cooling system's output is consumed to a large extent by the latent heat transfer required to condense ambient humidity. Specifically, about 1065btu/lb is required with a condenser coil at 50 deg F. But that's OK because...
In a vehicular application, the humidity is lowered quite quickly because the limited air volume inside the vehicle is rapidly recirculated through the cooler, providing what feels like immediate relief for the occupants. Now, this drier air is subjected to more efficient cooling (requiring less btu per degree temp drop) as it continues to circulate through the cabin. Lastly, the air in a vehicle is dynamic, meaning it's moving around, blowing nice (relatively) dry air on the occupants and keeping them cool, so the air temp itself doesn't need to be ice cold. And this is why....
I chose to go with the portable icebox solution. I just couldn't bear the idea of lugging a permanent setup around with the extra weight so far back in the CG envelope, eating up my useful load 100% of the time when I only want it for less than 30 min of each flight, and even then only on some flights, not all. Add to that the extra drag of the condenser intake hanging out in the airstream 100% of the time and well, you get the picture...
Results: One 22lb bag of ice gets me a nice comfy cabin on a hot sticky mid-western summer day (benchmark - no sweaty back) for taxi, takeoff and climb, lasts about 2 hrs turned off in cruise and provides another 15-20 min of cooling to cover descent, landing and taxi. If I have another leg to fly, well I - have to find more ice to be cool, but I'm OK with that and so is the wife (usually)
Now don't get me wrong - I want my air cold in my Toyota, and I'm not too pushed if it adds another 75lb or eats up a few HP or adds some cooling drag to my car, but not in my airplane - no way, no how. But that's just me, everyone has their own needs hierarchy