The bad part about putting a clear cover over the fogs is that they'll probably become so dirty & scratched over time that they'll eliminate any benefit the fog lights would have anyway... seems like you'd be better off with a removable cover (velcro, magnets, quarter-turn fasteners?) that you can quickly yank off for that 2% of the time you want to use the fogs... you wouldn't be going all that fast on those days anyway, so I'd guess the aero penalty would be minimal when the covers are off...
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