Just get a good wire, then right before you thread it in, smear a goodly amount of silicone grease on the first portion of the wire jacket. As it goes in it should make a nice fillet of grease to seal it. Don't use regular grease, silicone grease aka "tune up grease" for cars.
I've rode in the rain a time or two - my magneto case is decently sealed - but odd as it sounds, the engine will run better with the grease on the plug wire vs. without. I've tried it a few times both ways.
SBP Plug wire