This weekend i was in the process of replacing the fishfinder i had with a newer model and i ran into this situation when removing the transom transducer from the old unit. The screws had a nut on the inside of the boat in an area that is completely covered by fiberglass insulation or something like that, I was unable to remove the nuts so i ended up using a saw to cut the screws of, now i have two screw size holes that i need to plug so i was wondering what is the best way to do this.