You may or may not really need ventilation. All of my hives have bees that like to hang out, but they aren't really bearding. Sometime late at night, they go inside. If you do need some ventilation, you could try something like this:
from Betterbee. I use these a fair amount and on a couple, I've put window screen inside so that I can transport a hive and give it a little extra ventilation without losing bees. The plastic plug allows you to close it completely. I don't know if your feeder extends too low and would cover the hole but it's worth a try.
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