I wouldn't worry about it. The plate bears no load--all it does is cover the gearbox, and the aluminum one doesn't fit any tighter than the plastic one.
I have never heard of anyone having an issue with the plastic side plate on a Z-series Spinfisher, and they've been standard issue on the reels for at least the last 20 years.
If you're really worried about it--buy a beater greenie 704 and paint the plate black
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