Little bit of column A, little but of column Buttplug.

You're mom

In that case I'd vent it properly and make sure the whirlybirds are actually functioning(they often dont). Also you'll want to make certain the rest of the sea can is water tight(as best you can) if it isn't those roof vents won't be enough to keep excess moisture out.

Exactly that. Just make sure it's waterproof caulking and the screws are self sealing.

To address your other question, assuming it's sheet metal just cut the vent out, get yourself a piece of 13"x 13"(or bigger, just get a piece 1" bigger than the hole), a box of self sealing sheet metal screws and some waterproof caulk. Should do the trick and should keep out the water.

For those exhausts to be even slightly effective they need a wall vent to induce a draft, otherwise they aren't doing anything. But..I mean they aren't the most effective even when installed properly so..🤷‍♂️ since it's a storageshed maybe you don't care much about moisture inside? I guess it all depends on what's going to be stored in there, and how much work do you want to put in?