For this use-case, a condom is the best.
Use a zip tie to retain it but spit on the post first as an anti-rust inhibitor. But don't cut the loose end of the zip tie off. It will help wick away moisture to also guard from corrosion. And if you need to use it, use a tiny screw driver to release the zip tie. Then when done, you don't need to waste a new zip tie to re-secure it. Just to be clear, I'm not talking about using the post, I'm talking about using the condom.