Fiberglass and stainless will have different expansion rates in different temperatures so anything that just "pins" it in place (epoxy through holes, etc.) will likely fail over time. Bolts aren't the answer either because they can create very high compression stress on very small surface areas of fiberglass leading to cracking and failure. Wood is nice too, but, as someone pointed out, it suffers from the same same high compression loading from bolts and will compress over time.

Nacra catamarans built around the 90's (and I presume still today) use a method on the side stay chainplates where the chainplate is T-shaped with holes in the "T" section. Those holes holes are filled with long strand fiberglass roving. That plate and roving is glued in place and the roving is fanned out on the hull. Even if you get some mild separation of the steel and fiberglass, the roving that is woven through holes in the stainless will keep it in place. A Nacra 20 is shown below (and granted, this could have been made to look a lot prettier on the inside).

I whacked this boat so hard that the chainplate got a little loose. I built a pressurized cap and pushed some epoxy around the chainplate from the outside and firmed it back up.




I used a similar method to make a carbon fiber beam strap for a custom built A-cat that was starting to crack around where the rear beam connected to the hulls. Once the carbon roving entered the hull, I wet it out and fanned it out on the inside of the hulls.





You'll also notice the Orange N20 fenced in the backyard. That's "The General" and that dog was so mean it had to stay chained up.