[HaPPI]

The most efficient cooler is the one God built into all of us, evaporation from the lungs, sinuses and skin interfacing to the bloodstream, and what feels best beside. So think in terms of moving the air a little bit (so some kind of fan for one end and maybe the seat cushion gadget for the other) and drying the air in the car when it goes above a certain percent relative humidity. Some kind of chemical humectant that can be re-dried out at home in the microwave would be lots more efficient that current compressor coolers. No moving parts is good. Not sure about cost.

You'd want to keep the air recirculating to keep out the outside humidity. So the air temp might not be 70, but a dry, moving 90 degree airstream will feel cooler to you than a still, clammy 75, you crazy mammal you.

Oh, and tint your windows with heat-reflective film. And paint the car white if it isn't.

I once drove across West Texas in July with no A/C. The key was block out the window to keep the sun directly off ya, and keep the air moving. 110 degrees and it wasn't all that uncomfortable.