Jimmy:
My fasten method is by wood screws directly into the top layer of fiberglass; fully into the core, but not through the interior FRP layer. This is how the original teak handrails were fastened by Hunter.
Certainly epoxy fill is the safe-and-sound method. And I considered. But I whimped out on doing it. Two reasons. 1) For the forward most screw holes, there was no wood at all between the cabin top fiberglass and inner layer. Any epoxy injected into this area would have just flowed away. 2) When removing the original teak handrails, I investigated the condition of the wood inside the screw holes; all was sound. Hunter didn't fortify with epoxy. I figured that since the the original screws didn't leak for 27 years, I could do the same. What I did do was put in a slight bevel around the screw holes so that the caulk would be pressure-forced in when the fitting was snugged tight. Also for the holes where did I encounter wood core, I flooded them with penetrating epoxy liquid. The idea was to saturate the wood around the hole with the stuff. When dry, hopefully it will prevent any moisture from migrating further into the core. MaineSail, who is an expert on the "Ask all Sailors" forum, has posted that butyl tape is the way to go for bedding fixtures instead of caulk. You might want to access this for yourself.
One time consuming aspect of my project is that the bottoms of the fittings are flat, whereas the cabin top where I mounted is curved. I used a bench grinder to the bottom of the fittings to make the "flat" concave.
The reason I switched to stainless steel handrails is because I needed to repaint the cabin top. The gelcoat had oxidized through to the FRP. The teak handrails had to come off for faring, sanding, priming and painting. I had decided that maintaining teak to brightwork perfection year-after-year would not be a labor of love. And I think that unless teak is maintained to a high standard, an otherwise well maintained boat looks a bit dowdy and weather beaten. Also, I wanted the boat to have a more modern appearance. Stainless steel was the answer. And the price was right, both for material and that labor to install was "me-free"!