Pulled the battery box apart and made some flow diagrams.
Air is drawn in from behind the passenger seat , pulled through the battery, pushed into the battery box, where it exits through holes in the bottom into the spare tire area. From there, it doesn't appear there is any clear passage for air to pass through. On the driver's side, the foam almost completely blocks airflow. On the passenger side, air looks to most easily make it to the vent by the area around the pipe coming from the gas cap.
Poking my phone inside this hole, I found the louvers:
The space between the vent and the hole is almost exactly 25mm thick and perfectly fits a fan.
Passenger side seems blocked.
~~
Looks like the best place to put the fan will be on the driver's side between the carpet and the hole.