Are these flush mounted in a console? I'm having similar problems, see http://rib.net/forum/showthread.php?t=18114
I came to the conclusion that a cover would make it humid - like being in a greenhouse and may actually make it worse than if not covered for the instruments. If the cover is all-over tight fitting, there will be little room for air to get in out and any evaporation will result in a humid atmosphere under the cover.
In my case, I think the only thing I can really do is hermetically seal the back of the instruments in a box and put in some silica gell....