If I understand crystal clear then you want to set a small gap between top side of contour flanges.
You don't do this directly, but there is an solution.
Here is a video about this:
i think I have found an - at least for me - easier way to get what You are looking for.
Using the convert to sheet metal functionallity from ST7
Starting with a box, convert that with a gap to a sheet metall part, close the corne and finished
See the video:
Hope it helps