I was under the impression that it should be open at both ends.. Right now, it seems that any heat generated by the light, might make it's way down the tub you have and then just kinda get blown around at the end. Like putting a fan at the tip of a gun after firing it. Youd just blow around the smoke exiting it. Plus, you never want a fan pulling from a dead end. Go put a box fan with it's back against the wall and see how much air it pushes... not much.. plus it's bad for the fan itself.
You open the other end, it will pull through nice new air!
I am going to cut one 4" hole on one side of my box, and one 4" hole on the other... put
(O = the 4" hole)
(---> represents air flow)
--- O tube/cooltube/tube/fan/ O -->