If you put the stock on a slope wouldn't that mean that the angle of the pocket sides would vary around the pocket?
What angle do you need? You can purchase tapered endmills (intended for mould making in metal but they work OK on wood and plastic) in a huge variety of angles. These would let you mill the edge of the pocket in a single pass.
It would be very helpful if you will upload a drawing (or some kind of graphic) to show us exactly what you are wanting to accomplish. Lots of great “guesses” being made here, but we really don’t know what you need for sure.
I did it in Aspire using a 2-rail sweep just to establish what you wanted. Now it's just a matter of finding a way to do it with just tool paths so it can be done in Cut2D. Maybe others here can help also.