Out of an abundance of caution, avoid touching shared surfaces like signs or benches, and - yes - trailhead boxes. KLT will begin posting maps on the outside of trailhead boxes. Please prepare yourself by viewing a map in advance, and printing at home as needed, or take a photograph of the map at the trailhead.
This article in the New York Times discusses research about how long the virus is living on surfaces like metal or cardboard (potentially up to 3 days on surfaces like plastic or metal, but less than 24 hours on cardboard). They don’t mention wood specifically. Please take precautions and avoid touching surfaces in public places.