Anode ideas for spa copper heater

I have a spa that is heated by hot water passing through copper pipe under the seats. I have had problems with corrosion due to the chlorine and other water issues. I have coated the heater tubing with epoxy, but it still gets nicks, and corrodes. I think it would help to add an anode with some kind of clamp around one of the fittings. Does anyone have any ideas about the best material to use for the anode, and the best way to "make it happen"? There is a fair amount of room under the seats.
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