Tips for reliable card edge connections?
Card edge connectors can be ordered ruggedized, and with several platings of choice, check for instance Sullinscorp

There are ISA slots with a cap for protection.

Some vendors are quite willing to help you with selection for parts, if you describe your problem to them. 

You may want to insert an empty daughterboard into the connectors when they are not in use, if you cannot use a cap.  

Make your design so you can easily exchange the backplane too. 

Usually, gold plated contacts last a very long time. When they didnt, we used a piece of cardboard with a very fine sandpaper (1200 or higher) laid over one edge, and inserted into the slot once. But only as a last resort, back when industrial PCs where still expensive. Sadly, I found no documentation about the connector lifetime. 

Also, for extended outdoor use, you might consider a fully sealed enclosure. Filters tend to clog, nasty ants will move in, or even the dreaded micropenguin might build a nest inside the enclosure, if the climate is cold enough.