Had to cut my underbelly also to inspect the gray tank brace for the galley. Cut the opening in the shape of a "U" so as to have a "FLAP"
on the front edge. Used a black mobile home/ RV tape and a "SNIPE" sign to close the cut. The snipe sign is made of the same stuff as the underbelly. The only difference is it is white, has a "promise" to make you millions if you call the 800 number, you know the signs that clutter every street intersection. Used piercing screws from Lowe's to attach the sign after taping the cut underbelly. Then taped around the sign patch.
Love my Boliy PRO3600Si, weighs only 68#. Uses the same engine as the
Yamaha 3000. Just converted mine to a external run fuel tank. Installed a pulse fuel pump. Uses Yamaha generic outboard fuel connections and 3 1/2 gal fuel tank, will run 14 hours on one fill up. Has smart throttle, will handle my 15 BTU a/c with no problem. Stores in the front storage of my fifth wheel.