How to extrude a curved hollow surface

Hi I am try to make a hollow area on a 2mm thick polysurface but do not know how. I can make the curved hole on the outer surface but cannot extrude it to the inner surface. Any advice will be great, thanks

Hi Connie -

If I understand what you are trying to achieve, it should be possible to offset the top or bottom arc of the hole on the outer surface to the inside. Then extrude that curve and use the borders to split the inner surface. You might get a better answer if you post the 3dm file…