I'm trying to make a sheet metal part that's like a tube, but has a gap cut out of the bottom side of the wall. The faces of the gap on the tube's wall need to be parallel with the top plane as shown in the attached file.

This is how it needs to look from the front view (as shown in the file):

_____

/ /       \ \

\ \       / /

In the attached file, I've used a sketch on the front plane and made a base flange from it to the appropriate length. However, this won't flatten. I've also tried using an extruded cut to chop the corners off of arc and circle based Base Flange/Tabs and made the faces parallel with the top plane and I've tried using a lofted bend. On the circle based Base Flanges I've used both a complete circle and one with a small gap.