I am not flying my rotary yet but a practical solution to dissipate the heat trapped within the cowl (after shutdown) is to incorporate a Volkswagen Bellows thermostat.

(See it in action here.)

You attach a rod to it and as it gets hot it expands and opens a vent in the top of the cowl allowing cooler air to pass through using  a chimney effect draw from the escaping heat. Once temperatures get back to normal it shrinks and closes unattended.

T Mann