Mount a flat board on the backside of the shared wall by using through bolts with blind heads and backing plates.

To that board I can mount anything I may need to... by through bolting or wood screws. Over time, if that board looks like swiss cheese, it is easy enough to remove it and replace it.... using the original through bolt hole pattern.

This is something of a combination of the ideas I saw here... Steve set me on the right track, and Greg and others gave me ideas to further enhance the basic concept.

As for the waterproof aspect of the charger... it is fully sealed, filled with some resin.