My 16 OB, The Mule, finally sprung a leak in it's fuel tank.

In my boat the tank is under the splashwell all the way back. I am thinking I may just leave the old tank in place to avoid all the cuts and install a new belly tank in front of it. On the 16's you can pull up the 30" long floor locker and expose the keel. What products could I use to make a mold of the "curve" of the keel that could then be removed? My goal is to follow the exact curve of the keel and then use that base to give to my shop to create the shape of the base of the tank to use every inch of space. The only thing that came to mind was soaking a thin piece of 13" wide Luan Plywood in water overnight, then drop it down into the keel with a heavy round pipe on it and come back a few days later! Not the greatest idea I'm sure.

Anyone have thoughts?