I use the bottom of the engine compartment for a LOT of exhaust holes. I happen to mount most motors upside down for that reason. The small GWS Corsair has 6 small 1/2" holes drilled in a downward and backward direction. 6 small holes DO NOT weaken a structure as much as 1 big hole of the same area.
If you use offset gearboxes you can REALLY get the motor low & directly into the airflow.

Air flows from the inlet straight to the outlet. Even if a motor is directly in the way.

Think like that and you should come out fine.