An illustrative example embodiment of a structure for supporting a heating, ventilation and air conditioning (HVAC) unit includes a frame having support members configured to be situated beneath the HVAC unit to support a weight of the HVAC unit, which is directed in a vertical direction. The frame is at least partially situated in a plane that is perpendicular to the vertical direction. A supply air passage and a return air passage are supported by the frame. The return air passage has a first side and a second side opposite the first side. The first side is oriented at a first oblique angle relative to the plane. The second side is oriented at a second oblique angle relative to the plane. The first oblique angle is different than the second oblique angle.