Putting a thick washer on the balsa blocks on the side mounts can open up some air space - on mine it was about an 1/8 of an inch each side. That should help with airflow. I haven't flown mine yet, but prolonged full power ground runs don't seem overly hot. A little rich on the carb setting helps keep it cool too but we'll see first flight how hot it really gets.

The black pipes on the left side are real and I put fake ones on the right side. With some holes on the bottom but excluding the gap the washers created, the ratio of out to in is slightly in favour of out. I also used some small baffles on the inside of the cowl to control airflow.