Youll be fine mounting the charger there, and use maybe a 3/4″ long screw(stainless), pre drill the holes 1/8″ dia. and use polenty of good silicone to seal the screwholes under the charger.(you might even try mounting with just silicone ?) Those are floatation boxes, and are filled with foam, so you dont want any seepage into those compartments. Most of the current chargers on the market are waterproof and wont be hurt by moisture, but be sure and tie up the leads from the charger which are stubbed out for the remote gauge, as there have been some instances of these causing some issues. Good luck, and anything i can help with just holler ! D.