Try setting up a Dremel tool with a cutter as a router to remove the remaining deck. Attach it to a plate large enough to straddle the hull without it rotating off the shear. Secure the hull so it doesn't move as you move the router around the shear. Make the hole large enough where you can see the work. I think Dremel makes a tool fixture where you can adjust the depth of the cutter.

How's your project coming along bud. We're here to help