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Jester I used the rubrail backing at the top to secure to and the floor reinforcing rib at the bottom. I actually got the idea from Newty. I used #1/2-10 sheet metal as screws if I remember correctly. I also used a bit limiter on the floor rib so I wouldn't drill too deep.
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I definately wouldn't use metal either - aside from the risk of grounding out a stray power wire, you also will probably have some funky noise issues if you are running multiple amps.
A lot of people use cutting boards (like for cooking) as they are cheap, easy to find in all sorts of sizes and colors.
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Metal is fine, remember plenty of auto installs have been done on metal, it's not an issue as a metal rack wouldn't be chassis grounded anyway. There have been plenty of boat installs done on custom metal racks. Mine are done on metal without issue and there's no risk of funny noises as they're digital and don't create noise or movement. If you did have a stray wire (which you shouldn't) then it would just pop a fuse, that's what they're for.