I built a mounting bracket whenever using the Headrush cab. It was made from 1"x 3" wood strips using glue and screws and bolts to the top 17mm holes on the top of the cab. It provides enough side bracing to hold the 4U rack in place.
Also it tilts back a bit with a 1" x 3" piece of wood. This also provides a better view of the Kemper. At my age this is an easy setup w/ less load in. I simply hand tighten the bracket onto the cab and tilt it back after wiring. The mounting bracket has self stick padding so the 4U rack has a bit of protection and dampening.
The rear cover of the rack mount also is used to store the remote during load in/out.