I have 7ea of these 2 -1/2” Magnetic Wall / Ceiling Boom Positioning Devices. Mount the base on the ceiling, and the Puck on the boom and they work to center the booms when not in use. We use these on our Mosmatic Z Booms in our self serve bays. These 7 units have never been installed. They sell for ~$150 ea and I’d sell all 7 of them for $250 + UPS / FedEx costs. 
