My control panel is a fully enclosed box. It is mounted to the cabinet (UAII) using flanged insert nuts (
like these 
) and large head bolts (something
like this 
, but shorter, and with more thread).
The inserts are in the cab, and the large-head bolts drop through holes in the bottom of the control panel. Using 6 in all. Provides good strength, with easy removal.
The control panel top is hinged in front (player side), and the back (cabinet side) is held shut using something much
like this 
. They are hidden from view on the sides. Makes for a strong hold-down, and relatively quick access to the underside if need be.
That's my current plan, anyway. Open to suggestions.
As to the original question, top-mounting everything.