currently MoI does not have an option to create a fillet with a uniform distance between rails. So you have to do a workaround by first sweeping a circle along the edge. Then boolean subtract the swept surface from the original object. then delete the "scooped" face from the channel. Now use Blend to create a surface to fill the gap. and join the surfaces...

this is not perfect result for what you need. It's more like a "distance from edge" fillet. But it should be closer than MoI's "rolling ball" fillet...

jonah