I use some plastic barrel On Time feeders as secondary backup feeders at my stands. They come with plastic lids that clamp on air tight. My hanging ones in the woods I used deep dish pizza pans for lids because of the squirrels so I have never had a problem with those. More recently I got one on legs out in a windy open area that I wanted to retain the clamp on plastic lid. I noticed some condensation problems this season that I am sure would turn into mold problems with a slower off season feed rate. I saw a deer feeder breather kit online, but it is real pricey. On that same idea I simply installed a cheap valve cover breather for much less. Time will tell if it works.

Interesting. I'm having the same problem with my Texas Hunter Fish feeders. I might give it a try.

I bought the 3/4 inch breather off Amazon for $9 and it came with the grommet. The grommet took a 1 1/4 inch hole, but I stopped drilling it just short of that for a tighter fit.

Great idea. First feeders I built I made them super tight and weather stripped. Soon learned that was a bad idea; corn carries a ton of moisture.