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
If you've got short, stubby fingers and wear reading glasses, any relaxation you would normally derive from fly fishing is completely eliminated when you try to tie on a fly. ~ Jack Ohman, Fear of Fly Fishing, 1988