Do a google image search for "car window fan" (without the quotes) and it will show you a few commercial offerings and some of the things people have tried.

Now the bucket of cold water - cooling the cabin is only half the battle. The battery is also exposed to high temperatures from the trunk. There are two little holes on each side to allow the IMA fan air out. Unfortunately, if memory serves, they have little one way door flaps on them. So even if one were to mount a big enough fan to suck all the air out of the trunk too they would probably snap shut and create a vacuum. (I could be wrong about this, it has been a couple of years since I saw them.)

If on the other hand the one window fan/vent contraption was set to push air into the car, and the dash vents are closed, then that should in theory push the cabin air through the IMA into the trunk and then out. Probably it would take quite a lot of flow to cool the trunk. If you happen to have a thermocouple it wouldn't be too hard to test this. Put the thermocouple in the trunk and close it on the wire (hopefully the rubber gasket will be gentle enough not to break it), with the meter on the outside. Tape cardboard over one open window and mount a fan to it with screws and nuts, aimed in. Turn on the fan and take measurements on a hot day.

Sometimes I wish cars for our hot climates came with these mounted on the roof and trunk lid:

 

Folks have mounted a 90mm fan in the exhaust duct. It does flow a small amount of air.

 

The commercial fans I saw just had the window cracked with a small vent and the fan/panel inside. Also, reviews I saw said they don't work very well with tinted windows. I planned to build something more like this, with the solar panel on a fairly long wire so it can sit on the front dash. I'll probably also throw a smallish battery in there, just to be on the safe side. It'll be fairly trivial to build something to hold a replaceable filter on the inside.

Good to know about the fan in the exhaust duct S Keith, I'll check into it. Seems like I would be able to run the wire into the passenger compartment, and throw a plug onto it so I can power it with my solar fan.