Need help cutting a odd shape out of a curved feature

I am trying to cut an odd shape out of a curved boss and I need to keep the wall thicknesses equal.  One section, the outside edge of the cutout will leave me with a straight/sharp edge when just cutting using the top plane.  I think I need to either project onto the surface and then cut it out (Not sure how to get it to cut out after projecting) or I need to perform something else to get this done.  I have attached a few pictures of what I am trying to do.   The second picture, the outside of the cutout is the area Im focused on.  As you can see, cutting this way would leave a straight wall and I need it to be curved and not angled.   Any help is greatly appreciated.SolidworksParts And Features