Hello jetblast,

Please see the attached image. First create a rectangle, fillet the corner of that rectangle closest to the edge you will be using to create the filleted corner and center and align the rectangle to that seam. Run the Sweep command using the rectangle as the profile and the arc edge as the rail. Before completing the command make sure to select Both Pointy under the Ends tab of the Sweep command. You should be left with a shape that looks similar to the one pictured. Select the two outside and two inside faces and delete those, leaving the fillet shape. Run the Trim command using the swept fillet as the trimming object against the solid. Hide the swept fillet, and delete the two trimmed faces. Unhide the swept fillet and Join the fillet to the rest of the object to create a watertight solid. I admit I was thinking about this a while to come up with something that worked, it might even be interesting to experiment with scaling rails on the sweep to see what other shapes are possible.