suck on the inside ,blow outside.
keep a eye on the temp,using 400 hps .in a confined space like that,it will get hot.i also have a small confined growroom so i have the door open a little during daylight hours.i dont use extractor fan & my temp went to 42 degrees.the leaves started to curl & dry out but i managed to save both plants (PHEW)
oh yes that was using 2 400watt hps.i only use the 1 now.