its box built in middle of room in basement intakes are by the floor exhaust is into the room but out top of box.. all walls are plywood except the one that i open to do stuff inside is just a sheet of panda poly with a tarp zipper to open/close it duct taped all around except the bottom is velcro.

heat has never been an issue its just that the negative pressure/suction sometimes loosens the duct tape holding the "door" up that leads into the box. box is 60inx48inx72 in and is raised about 10 in off ground so inside space is only about 60x48x62 (lxwxh).. i havent been able to find any 12 in circular duct or elbows at home depot so that is why i ask about light proofing a squared cut of 10x10 in or any other dimension equaling 100 sq in

hope this clears up my problem, if not let me know what you need to know