Hello! I have a rare, used JMR Electronics ProBracket rack mount with sliding cradle for mounting a 2013 Mac Pro, for sale!
Used this myself for a few years - these are great for keeping your Mac Pro cylinder out of the way, off your desk, and in your rack.Requires 4 rack spaces of room in order to fit, and it can be mounted on either side of the rack.
Comes with everything pictured...rack hoops for holding your Mac in place, rack sled, and two mounting screws for going in your rack.
Also two velcro strips for the cables that'll be running along side it.The rack hoops have some cut-out patches of padding strips still left on them - if you take them off, make sure to replace them with new ones. These hoops will scratch your Mac if mounted without the padding.Feel free to reach out with any questions!
Thanks very much, and good luck!