Hi .. I'd like to make a side reinforcement in a box edge similar to the one in this cardboard box:

I've tried making a cut out in a bent main piece and then having edge flanges return, like so:

this works fine, but I can't figure out how to connect the bottom pieces back together and still be able to flatten / unflatten the part.  Using "Up to edge and merge" as the flange length option doesn't work because it requires the merging item be a different body.  Using an extrusion the thickness of the metal doesn't work because that breaks the unflatten ability.

Any ideas?