Use an Arduino Mega in a incubator (high humidity)
"-40 °C to 85 °C" for the processor, yes, for the other parts, who knows?

The board doesn't get that hot unless you are powering from like a 12V source, and then it's just the regulator that gets hot.

You could a spray-on conformal coat, Mouser.com carries them. Insert any wires into the headers you will use and cover any headers you may want to use before you spray, if you get coating down into the female headers you may have poor contact later on.

The ziplock bag idea sounds like a good start as a start, put some silicon along the seal to fill the holes where the zipper won't close right next to the wires.