How to install hardware on cabin top when I can't access the underside?

[Maine Sail]

Alex,

Just don't "seal" anything on the inside of the boat. The bedding compound should only come from the outside. By doing this you allow any leaks to drip into the boat and be seen and repaired in short order. If you seal under the fender washers/backing plates for example you will only force water to be trapped between the deck skins which with stainless bolts can lead to crevice corrosion or with an un-sealed deck more rot and moisture issues.

Another option I have used for cover plates, say for a winch, is 1/8" or 3/16" Lexan. I use a router to round over the edges then wet sand and heat treat them for a factory look and finally use Krylon Fusion to match the interior color. You can also use Lexan for mounting instruments to old holes that are larger..

This was for some ST-60's in old Datamarine holes. Matte Black is the Fusion color.