Developed a lot of tools for punching large diameter holes in sheets of plywood; many of them are rather complicated or cumbersome. I need only a wooden plank to two nails. Hammer them into the tank so that one became the axis of rotation, and the other is a kind of cutter protruding from the straps on the thickness of the plywood. Zaglubit in the leaf axial of the nail, start to rotate the bar, pressing it slightly, the second nail will begin to scratch the track is a circle of the desired diameter. Below the surface was not chips, you get two with the watchman. Within a few minutes you will get neat holes.