I dont know if I would call that a fillet. More like a constant variable blend!
Here I did your model and used some trimming to allow the blend tool to work.
I also altered the model in a way that allowed the constant fillet to make a result, then did some lower level work to finish it off. Here's a link to that one:
Both methods though, are left with needing the "n-sided patch" to get a quality surface. Something on my wish list. With that tool, there will be most nothing that MoI could model!
