i have close to same setup. just cut like a 6"x6" passive intake in that and you should be fine. whats the cfm of the fan? thats probably the biggest concern.

the way i went about venting both veg and flower chamber is i got a 450ish cfm squirrel fan that pulled from both sides of the unit. i mounted it in veg chamber blowing out the side of the closet. drilled a 4" hole so one side of it could pull from the flower. and one side from the veg. then my cool tube has a separate 150 cfm fan exhausting air throe the chamber.. i have 8"x6" passive intake in both compartments with grills and filter on em. and small circulation fans in each chamber