Just remember, don't open your windows all the way. Only a little bit. When you open them fully it creates a lot of turbulence and increases the size of the wake big time. I remember when tuft-testing my VGs when I opened my windows all the tufts of yarn went all wonky because the mass of turbulence the windows created wrapped around the car.

If you are worried about adding hot air via the heater core, you could put a manual shutoff valve on the inlet line from the engine, and ensure that no hot coolant is flowing to the heater core in the summer. :)

I just leave the knob turned all the way to cold...

Me too...luckily, my car has a manual button for the compressor, so I just turn the climate control all the way down, and if it gets cool enough, I can just turn the compressor off until I want it.

Mmm-hmm. Me too.

Run AC full cold on recirc if you really need it, and turn AC part off/on manually leaving fan on 3rd position.

AC ruins gas mileage

Or never use it and you WILL get used to it eventually

Every water-cooled car I've seen already has one. It's generally cable-operated by the temperature control just like the air mix flap, though some cars have a vacuum or motor operated valve.