Like you would just make your enclosure out of the zinc is what I am thinking. I know satellite dish parts are often made out die cast zinc and those are out in the elements for years. For instance here is an example part made of a zinc casting: https://www.dynacast.com/en-gb/specialty-die-casting/die-cas...

The only thing about metal enclosures is you just need either a connector passed through for your antenna or a channel that your antenna can be potted into like on some cellphones with an aluminum body. Since an antenna in a metal box is umm not going to get much if any signal.

--Edit-- Also you can find lots of old radios before plastic was wide spread in metal enclosures. Although those old devices are not exactly water tight no gaskets or sealant between parts. Unless you look at older military grade units.