Since the original sketch was simple, I recreated it on the inside dimension of the open end which allowed me to construct a profile which I could select and extrude to match the other 3 sides.
For a more complicated sketch/body I'm sure there is a better approach. If you know of one please comment but I've resolved my issue.
Update: I followed Edson's approach of recreating the sketch from the plane with cutout, extruding the walls up and then applying the fillet to the body which was the key. Prior I had only applied fillets to sketches and then extruding. Made all the difference.