Well, I'd get a tent. I used to make or modify cabinets, but then tents came along....lol, but I don't think it's going to be a solution to this problem. How much volume does your fan move (cfm)? I agree on testing without the filter. Is there a source of fresh cold air somewhere that you can run a duct to? I just punch a hole in the wall down low and install dryer vent so it looks normal from the outside. When I move out, just patch up the drywall and leave the dryer vent part outside, but seal it up. Nobody has ever even noticed after I've left when they are outside that the vent is out of place. You see them outside all of the time.
The end all be all solution if you really want to get this thing bangin'....
I would consider using that fan to power an intake duct and get a stronger fan to exhaust with. That is of course assuming that this fan is too weak to move enough air through the cab. A bigger space generally means a bigger fan so I don't know that you want to go bigger yet.
I don't recall, did you buy a dimmable ballast so simply switching to a 250w bulb will work, or dimming the ballast on your current bulb?