A year on the banner would be pretty cool, but it would also make them much bigger...Did you rub your fingers together like Monty Burns when you said "Excellent"
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Banners should be removed at the end of the year, or if they have to keep them, put the year on it. eg: Tank of the month winner 2020.
Maybe have 11 tank of the month contests, and the 12th is a competition between the winners from that year.