Been working a bit with contractors that need a more accurate representation of duct. One client is needing flanged duct system, so I ran a little experiment. I created a union from scratch (since the one that came with the program didn't really represent a true union - no union is solid, it's a void). I created a simple solid rectangular form with a void in the middle. Next I added reference planes for the height, width and thickness of the the connection. After these were defined and dimension parameter labels added, I added a flange dimension parameter. All of these were instance based.
So here's what it looks like in the family: