An apparatus cools electronic circuitry. An enclosure surrounds the circuitry. Air intake holes are in an upper portion of a first panel that faces a first direction. Air exhaust holes are in an upper portion of a second panel that faces a second direction opposite the first. Air plenum piece is disposed within the enclosure, and a substantially planar portion extends from a first panel inner wall and ends a distance from a second panel inner wall. At least one tab extends from the substantially planar portion to an upper panel inner wall. The air plenum piece divides an enclosure interior into a first volume, enclosed by the first panel inner wall, substantially planar portion, at least one tab, and upper panel inner wall, into which the air intake holes open and a second volume into which the air exhaust holes open. The second volume is a remaining interior volume.