Small world. I have recently removed our winches to add some risers under each one. As a part of the project I will put some G10 plates beneath -- probably not needed... just seems like a good idea. Either 1/4" or a tad thicker should be plenty strong. The new plate will be "bedded" against the underside with thickened epoxy.
Be sure to over-bore the old holes first, fill with epoxy, and rebore.
I hope that you do not find any water intrusion in the coring.
Loren