Buy a couple of the large, quart sized perforated canisters full of silica gel pellets. I have two in a 12 gun cabinet. Once a month or so I take one out and put it in a 330 degree oven for several hours, at least four, and this does the trick, keeping humidity at roughly 40-50% for four weeks or so. Once my Bluetooth hygrometer shows it creeping towards 55% I repeat the process with the other canister. I have a heating rod too, which isn't terribly effective but does seem to lower humidity a reliable 3-4%.

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https://www.amazon.com/Dry-Packs-Gram-Silica-Canister-Dehumidifier/dp/B003QZ6PZ0

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I live right on the ocean, so this should work for you too.