Fred:

Sorry, I suppose this is a classic example of too much over too short a time frame. :^)

The "ideal hole diameter" in the firewall must be (3) three times the inlet area of the smile inlet on the front of the cowl.

The ducting cross section "area".... mean cross section back to the air exit has to be equal or greater than (3) three times the inlet (smile opening) area. Anything less will typically result in eddy of the air flow with minimum exchange of air in engine compartment.