Use stand-offs to hold the coaxial cable away from the tower leg. There are no special mounting requirements. Some balun manufacturers will tell you that their baluns have built in lightning protection. Those that do, use spark gaps which are absolutely useless. All critical components are completely protected even if water enters the balun's case.
Moisture can enter the balun case only through the holes where the wires emanate.
You can completely seal your balun by putting a small amount of CoaxSeal around wires leaving the case. Press the CoaxSeal firmly around the wire and against the case. This will insure a weather tight seal. Always protect all coaxial connectors with CoaxSeal. Seal all electrical components and coaxial connectors exposed to the weather.