I used to have a small waterproof box (well, it was when I started) on the transom with a handheld GPS mounted to the top. The box contained a small terminal strip and fuse block, and fed a couple of cigarette lighter plugs in the bottom (so as not to catch and hold water) for power take off for accessories.
Not ideal; to see where you were heading to meant turning around and looking down. If I had to do it again, I'd build a small "console" on the opposite side of where I sat and mount stuff there.
jky
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