Re: Installing a new winch
A bigger issue adding winches to cabintops after the fact is the likelihood of crushing the balsa core, esp at the 'pressure points' of the bolts and small washers. A backing plate helps spread that out.
Seem this might be a good place to drill out a larger hole (perhaps leaving one skin intact, fill with thickened epoxy and then redrill the holes. This would leave solid material in way of the mounting bolts, eliminating the crush risk, and at the same time thoroughly sealing the core.
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)
1984 Fast/Nicholson 345 "FastForward"