Putting the fan right over the exhaust will create the vacuum/increase the vacuum effect Making sure the exhaust has proper air flow to do its job is essential, remember how a car will shut down if the exhaust is plugged? If your exhaust on your box doesn't have sufficient airflow, the fan is just a fan pointing at the wall - So make sure it has room to vent. I don't have this problem because I use actual intake and exhaust venting. A forced air intake would also increase airflow. Excuse my rambling, enjoying my last crop Okay, so you only need to change a few degrees, which isfine, just moving the fan should do that for you. Remember, if your using foil, and not mylar, your not reflecting just light, but also heat. That's why we use mylar, because of the high light and low heat transfer. You can also use the insulated mylar which many of us CFL growers use, but it won't help your box as much as it helps ours.

Oscillating fan is the only other thing I can think of which will help your temps.