Hi Joe, is is possible for you to post a 3DM model file of that?

It's probably going to be a difficult to put a fillet on that, since it looks like it has some areas that go through some tight bends - that tends to cause fillets to have problems.

One possibility might be to build a tube along the top edge, trim it to cut a hole and then use Construct / Blend to put in a blend surface.

If you can post the model file it would make it a lot easier to do a few experiments and possibly give a better suggestion.

- Michael