1.   A good waterproof adhesive should be used in addition to fixing screws.

2.   And I cannot imagine there are not any waterproof adhesives around.

3.   If the work will be exposed to the elements, a waterproof resorcinal adhesive is a good choice.

4.   Invented by the military during WWII to keep ammunition boxes dry, it was a khaki-colored cloth mesh tape with a rubber-based waterproof adhesive.