The real problem is not outlet /inlet size.
calculate all you want --
You have to have a perfectly sealed - single path from inlet to outlet--plus- directed flow THROUGH the fins.
In order to make the air siphon (that's what is really happening) -- the outlet must be in a low pressure area -or-a low pressure outlet must be created along the skin of the fuselage -using a lip/dam whatever you want to call it .
Honestly, the fin orientation is awful on that engine setup for good flow.
On full scale craft --where airspeed in much higher ( and air is operating in higher RN setup) you can use relatively tiny inlets and outlets -- not so with this model stuff at small sizes
basically the air is reluctant to flow along the walls of the cooling path and TOTAL flow is greatly impeded.
Think of this
If the air in a pipe -sticks to the wall in a 1/8" thick layer -- a small pipe -say 1/2" ID --has a hard time flowing any air
A bigger pipe has the same 1/8" layer of air sticking -- but the larger ID lets RELATIVELY more air flow thru
In your case you have the worst of all situations
small size , and a convoluted path.