I'm still new at this. Read what I could find on the forum and manual.

I'm trying to get a round fillet all along the top edge of the cut-out where it meets the top surface.

I joined the surfaces and started with small 0.02 fillets - no luck.

In case I need to use a different method to create this model, here is what I did:

Extruded an oval with one flat side along a straight line rail to obtain a curved-top rectangle solid block.

Extruded the 2D cut-out shape and subtracted it from the object above to create the cut-out.

Joined the top surface and walls of the cut-out, then attempted to fillet.

3dm file attached.

Ed