Is the inner box a double wall on that? The iner will be prettty easy to get the 1/2 cicle once you have decided ont the height, getting the outer lined up will be tougher. Remer like pat said cut small as you can alway finish grind bigger to get a nice tight fit,Any thoughts from anyone on trimming/flanging the outer(to outer 1/4 panel) part of the tub and using durabond to attatch to the outer potion of the 1/4.