Had the same problem when I replaced my Furuno with a Garmin GMR18 closed dome. Made a round piece of starboard - rectangular would be ok too. Installed blind stainless nuts in it so it could be bolted on from under the hardtop. Made the new holes for the Garmin in the starboard (use 1/2" material) and recessed them for the Garmin hardware. You could use flathead bolts instead but I think they are metric so you would have to order them in.
Last year I refinished the hardtop and installed a new Garmin Fathom GMR18. Old holes filled and new ones made.