Hi Steve, also another thing that could possibly help you is if you select the 2 surfaces that share that edge, and use Edit > Separate to break them apart from the main solid, but they will still be joined to each other.

With that situation the filleter no longer has the complexity of collisions with the surrounding side surfaces to try and deal with for filling in holes, and you can create a fillet successfully similar to surface/surface fillet that way.

It will be a kind of long extended piece like this:



But it may give you enough raw material to work with and cut down.

- Michael