Great idea well executed
I've done a full review for the main stand so just a short one here.
I've used this with the 18813 arms. The 18819 tray is very well made from a heavy duty sheet of metal. It connects positively with the arms and does not slide about. There is a cutout handle for carrying the tray and a material covered lip on the front edge to stop items rolling off. It is plenty big enough for a PC keyboard, mouse mat and mouse so can mean you can use the keyboard stand as a workstation for your DAW. The front lip is rounded off by the material so it is comfortable to rest your wrists on.