Another option is to add a temporary exterior cleat, and clamp the deck or staple it to that.

Simply tape off the hull, and staple a 3/8" x 3/4" strip to the outside of the hull. Then shape it at the same time you shape the inner cleat. Then carefully tape the outer cleat only, goop it up and attach the deck (which you have cut oversize per the directions anyway). Staple the deck to the exteroir cleat.

After the goo goes off, pull the staples out of the deck, and rip the cleat off the outside, staples and all. Then trim the deck and cover the staple-holes with a rub rail.