Abstract: A room air conditioning unit may be mounted in a window opening using a device, on each side of the unit in the opening, having a straightforward two-piece construction. The first piece is a molded C-shaped frame which straddles the unit from above and below in the opening. A side member of the frame is abutted against the side of the window opening. A lower portion of the frame rests upon the bottom of the window opening, while the window sash is lowered down onto an upper portion of the frame to participate in securing the unit within the opening. The other piece of the device is a panel having a Z-fold central portion. One side of the panel may be attached, for example by screws, to the case or housing of the room air conditioning unit. The other side of the panel self-locks within the side member of the frame, using a locking mechanism provided therewithin, to close the space on either side of the room air conditioning unit within the window opening with the Z-fold central portion of the panel.