Make a brace across the top of the tank from one side to the other, glass a fixing point to the bottom (or floor if it goes that far back) each side of the tank. I'd make the brace 'bolt down' so you can remove it to get to the tank.
Have a fitting made in the middle of that so you can remove the pole, such as a cup for the base of the pole to sit in, with a pin through the middle.
Drill a hole in the deck behind the rear seat, reinforce it, maybe bracing it to the transom to stop flex? (not sure if thats needed) and then slide the pole in through that to the fitting below
I may be totally wrong but that's how i'd do it, and it's the best suggestion so far!
James
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