An electronic equipment enclosure includes a frame structure, one or more enclosure panels mounted on the frame structure, an exhaust duct, and at least one adjustable filler panel assembly. The frame structure and the one or more enclosure panels together define an enclosure. The exhaust duct is substantially the same height as the enclosure and has an air inlet opening formed therein for receipt of exhaust air from equipment mounted in the enclosure. Each adjustable filler panel assembly selectively blocks a portion of the air inlet opening in order to prohibit air exhausted into the duct from flowing back into the enclosure.