A hot/cold aisle air containment system includes a frame structure, at least one electronic equipment enclosure installed at least partially within the frame structure, and a header panel assembly. The frame structure adapted to be at least partially covered by one or more panels to define an interior space and includes at least one horizontal member. The header panel assembly is installed between a top of the at least one electronic equipment enclosure and the at least one horizontal member of the frame structure to prevent circulation of air above the enclosure. The header panel assembly includes a pair of panels. A first of the pair of panels is adjustable horizontally relative to a second of the pair of panels, and the pair of panels, together, are adjustable vertically relative to the least one horizontal member.