The best method is to replace the rotten coring in the cap. If it's in a place where you can reach it from underneath, you can cut away the bottom laminate, remove the old coring, glue in a new core and laminate over it. It will be a royal PITA, but not as bad as removing the cap. If you can't reach it from beneath, this may not be the "best" approach anymore.
Dave
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Blue Heron Boat Works
Reinventing the wheel, one spoke at a time.