I mounted one under the dash on that metal support, and left the screws out of the underdash trim panel. Because I didn't wire a switch into the pillar SPA gauges, I sometimes need to pull the fuse for them, to program them without turning off the engine.
I've another under the hood, atop the factory fuse panel. The wires and relays I've added since then make it a little tricky to get the factory fuse panel on/off, so I plan to relocate the added wiring and fusebox to the other side of the engine compartment, where the factory airbox used to be.
-jon-