hello, I need some help contouring a pvc tube fitting to fit the od of an 18 inch diameter tube .I have done plenty of 2d work nut no 3d stuff . basically I would like to fixture a 2 inch diameter pvc tube fitting with an OD of 3 inches on my cnc router and use a ball end bit to cut a contour on the fitting to match up to an 18 inch diameter cylinder kinda like a fish mouth . can anyone help me with this ?

Thanks