Maybe throw pvc pipe instead of the metal and there must be plastic housings for the outlets? Then add the cover to that and we should be able have it secured?
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You can definitely get plastic housings at Home Depot or Lowes and of course pvc piping. But you then have to still worry about salt creep and humid inside the outlet...only way you can solve that is isolating it as much as you can. Plexi with robber grommet boxes should work. Look at marine/boat outlets and see what's available.
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