well, here's what I would consider trying.
first take your plants out.
then the lighting, then everything else
then get 10 r 12 ft f panda plastic, duct tape, a staple gun, staples, and some srap cardboard, and carefully cut out and apply pp to the inside. make sure the bottom piece overlaps several inches up all the sides. get the pieces all in and snug with duct tape, then use the scrap cardboard to place on top of the edges of pp and SSTaple in!

then, use the bottom hole for intake. what are the dimensions of that rectangle? can you cut a 4 or 6 inch hole around it! go 6!
install yur fan on top, 4 inch hole? get the corresponding vent flange for the hole.
attach flexible ducting to both, done.
don't want the hole in the top? put it in the back upper corner.


this way you get yor hot air out and draw in cool fresh air. and pull it trough the cab naturally, and exhausting the heat from lights as it naturall rises.

then reinstall your lights put everything back, get a way to check ph, get cheapo plastic nurser pots, pack of keg cups, some kind of plastic tray to put on the bottom, maybe a grill, or sturdy wire grid to go on top. then put holes in the pots bottoms, up the sides, and when you water runoff can drip off into the tray and collect for easy removal and testing, and your pots aren't sitting in it.
that's what I would do first.