If there's other ways to do this, I couldn't figure it out. So far this is what's working for me. If it helps someone else down the line - great. I'm ready to install my new to me 'wart' once I get her out of the shed.
As a final 'goodbye' gesture to my old Norcold 3-way, I took the time to rip off the old fridge roof-vent, too. A handy chunk of 1/4" HDPE covered that baby over nicely. Really does, as Stephen pointed out, un-clutter the roof considerably.