hot air rises.
so you exhaust at the top of your cab...

you always need to provide your cab...with fresh cool air and cool air is lower to the ground...So, fresh air intake should be at bottom...exact specs...don't matter. cool in: bottom........hot out: top
Whoever said you wouldn't need a fresh air intake is an idiot, and knows nothing of controlled environment issues.

try that and it should work...You might be better off with just an intake and exhaust and forget the whole cooltube effort...See if that works. Goodluck