I can't seem to figure out how to add a fillet to this top edge here: 

I have tried:

- redrawing the spline used to create the cut.

- making a thickened surface of the cut and of the top of the model to fillet then subtracting it from the body (fillet wouldn't work)

- performing the cut as a loft between surfaces, thicken then combine subtract, thicken extrude, then adding fillet.

I'm running out of ideas on this one

I have cut this section out of a larger model