Just wire up any of the 12V LEDs for cars; or any 2V LED (most generic 5mm) in series with a 470ohm resistor (or parallel strings, etc; there are plenty of DIYs online), and just RTV all the connections; that should weatherproof most everything.
Hell if RTV can survive and seal up electronics for the DO-160 Salt-fog test, it’ll survive anything you can throw at it. (Test for any aircraft or ship-board electronics for corrosion due to water, salt, and sulfer; one of the toughest tests for electronics to pass)