Get a large piece of cardboard & trace the rear of the boat .Cut it out & verify the fit ,mess up get another piece of cardboard & retry. Once it fits right you can lay it on the wood & trace it out then cut & bolt it into place. I'b advise to get a piece of sheet steel 10 gauge or 7 & make a piece 1 foot by 1 foot on both sides of the transome . It is for the motor mounts ,so they do not grind a whole thru the transome as you apply & remove the motor.You run long bolts all the way thru witch also adds strength to the transome.
http://www.bucketrack.com/THEJig.html IF IT'S WET...IT'S CATCH'N FISH