ok so what u do is....

you have to make 2 selves an inch apart from each other, then u put a vent at
one end "lets say the lefts side" on the top shelf. then on the shelf below that
one, which will be the roof of ur other chamber, put a vent on the right side.
so u will have roughly a 3 foot gap in between the vents. then u can seal it
with calk, u can cover up the front inch gap ur going to see between the
shelves with white poster paper

so you pretty much are making a 1 inch 4 foot air duct

then put a passive vent at the bottum and mount ur fan at the top next to ur
light

problem solved

let me know how that works