My horizontal cabinet (was) simply the whole case dropped in the cabinet with an L-Bracket and a thumb screw to hold it in place when I move the cabinet around.
My vertcal cabinet includes a removable 2"x3" piece of plywood mounted to the inside of the cabinet, I have my motherboard, power supply, hard drive, etc. mounted on the board. A much cleaner setup IMHO.
(NOTE: photo was taken before I cleaned up the wiring)

As for the power switch, I wired directly to the toggle switch that came with the cabinet and set my BIOS to fire up on regain of power. I flick the original switch, everything powers up....when I am ready to power down I shutdown with my front end (MAMEWAH) then turn the switch off once my trackball light goes out (which is attached to the PC and indicates when power is off).
I also wired 2 arcade buttons to the top of the cabinet...a power (which I RARELY if ever use) and a reset switch....just in case...but they are used very infrequently and my new horizontal cabinet will likely not include these two buttons.