This is interesting to me , very very interesting. Maybe I should just have the ballast inside the cab then. If my temps increase, then ad more extraction and airflow. Stealth is a big issue for me, I can't have my landlord looking at a ballast with wires running into a cab he will think WTF and ask me what that is. Imagine growing right under the nose of someone and getting away with it. That is my situation. It works fine with my CFL's I have it dialed nicely now. But I will need to add to a hps in a couple of months for flowering.
Can't tell you how much I appreciate your reply, you have been the most help to me thus far. Others just keep telling me why it is a bad idea to have it in the cab, somehting I already know, but it will be necesary for now unfortunately.
**EDIT**
Had another look at your pics and I think I understand your setup better now. The end of your cooltube is still open inside your room. Here is my idea, connect ducting to the open end and have it connected to the outside of your cab. So the air is sucked from outside through your bulb and back out again. Then have another extractor sucking air from inside your cab and out. This way the heat from your light can't escape inside your cab, that should lower temps for sure I think.
Also a fan right across the plant blowing the hot air away from under the light, and lift the lights slightly, they are still small plants and you have a 600watt hps so they should still get plenty of light, all of this should help I think.