Good question..
As they say in the ad, the grab rail is threaded for a standard bolt pattern. I drilled a pilot hole in the console where I wanted to mount one of the bolts to see how thick the fiberglass was. Then brought the rail to the hardware store to find a bolt that could accommodate the thickness of the fiberglass, the biggest stainless washer I could find and a lock washer, and still have enough thread to come just shy of bottoming out the threaded portion of the rail.
As usual I probably bought one size bigger and smaller bolt, and some extra stainless washers just in case my measurements were slightly off.
Other than that I used some marine sealant for the bolt hole after drilling the correct size holes.
-- Tom