Don't bother with the Mesa switch or rack mount kit. There's external switching jacks on the back of the amp, use those. If you're racking it and using it with effects, grab a Nux PMS-2 switcher. They're cheap and midi, take the stupid rubber side plates off and they fit in 1U of space too. I have one, they're awesome.
As for the rack mount, I literally made one for my Mark III (which Adam now has on his Mark IV) out of a 4U rack shelf turned upside down. I just cut the length down with a power saw and drilled 4 holes for the mounting screws. Adam refined it with a 3U vented blanking plate for the front.