What kind of service panel to use when the enemy is condensate, not rain?
As requested: 
I have used small fire rod heaters in the past where condensation was a problem, 
they run off of 120v and are designed for a gun safe. 
I don’t know if they make them small enough to fit in a 40 amp panel. the little warmth they generate keeps the panel from condensation issues. I have done this in pool areas and open metal buildings. 
The first time I did this I asked the inspector, since then I have been asked what they were and after explaining have always been approved on inspection.