Customer designed some "sheet metal" parts but couldn't get a flat pattern. I am trying to re-create the part as sheet metal so I can flatten it for the sheet metal shop. I decided to extruded the base, shell it then convert it and add the flanges I need. My problem is the bends are supposed to be welded and I'm not sure how this should be modeled. Notice below the way it's modeled, the corners are all sealed (welded).
Here's what I've got so far (below and attached), but I'm running into trouble with the corners. First time I tried it I filleted the outside corners then shelled it to get the correct radii on the inside but it yelled at me when I tried to convert it to sheet metal.
I took out the fillets and got it to work, but now I don't know how to fill in all the corners.