With the best will in the world, your gonna have a task to try and waterproof that!
I would do the best you can with small dabs of silicon and some WD40 to try and keep the moisture out.
Best thing you can do is move that solenoid into the engine bay, as high as you can get. Dont go nuts with the silicone though, you dont need a lot, just enough to seal it. Remember you will have to undo those bolts at some point and if the sealant is all over it, it will get very boring.
Long term - save for an albright solenoid and keep maintaining it, if you dont, it will let you down when you need it.