I need some help here.  I can't get the "Welded Corner" feature to work on this part.  I want to add (4) welds total along the edges highlighted in blue here, at each of the four corners of this part:

(the short section above and below it do not need to be welded, I'd prefer to leave them not-welded so that they can spring open or closed a little bit when the guys bolt it down.)

I get the following error when trying to use "Welded Corner":

Any ideas?!