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Willy, if you build a damn in the mast, you'll still need to place a drain hole above it. And then worry that unless the dam is concave shaped on top--something like a balloon would invite water to pool around the top of the damn and corrode from the inside.
A small dam in the bilge and a small bilge pump, with small diamter hose, to keep the seepage out should do the trick without going into the mast or embedding the wiring in it.