You are quite right JLT, but if you read the Photo 4 description, you will see that I did advise that the cooling hole should be drilled out in TE stern post.
Actually any exit hole for cooling air should really be twice the size of the inlet vents, but any outlet is better than none.
I intend cutting another elongated hole in the belly under the wing to increase the exit vent area.
Your bottom hinge rudder mod. is a good one though, especially highlighting the need for an exit vent for cooling air.