It wouldn't hurt to get some ducting and an inline fan to bring fresh air from outside, get circulation, and most likely lower temps by at least a degree or two. Small price to pay to get the outcome you desire.


Outside wall > 4-6 inch ducting > inline fan (blowing in) > ducting to near plants > oscillating fan to blow from ducting to plant area. I guarantee this brings temps down, while giving you good co2 level, plus good airflow.

No matter what, you are going to need to vent the hot stale air to the attic, or out the window, especially if you wanna lower the temps and keep a good fresh air ratio.

400W is going to give off heat, in an enclosed area like that, cooltube is really all you can do to really bring it down.