I love all the ideas shared. Yes they got my gears turning again.
On our first motor home seen here, it came with a privacy curtain that attached to the decorative buttons. You can see 4 of the eight buttons with the decorative caps removed.


In the case with our PC, those buttons are used as seen in this picture.

I think adding one more button to the right on the green triangle would make for an ideal snap on curtain. Maybe a button in the middle and one at the far right for a nice balanced look. You simply use a teaspoon to pop off the decorative cap (green cap in my case here) and behind it is the male portion of the snap. With two snaps per side that sort-of follows the wing wall, that might offer enough support and enough of a seal. I am currently at work, unable to determine if anything is in the way. Maybe cover 50% across the triangle would be best to avoid cabinets, dinette, etc.
As far as what material to use, our old rig was upholstery vinyl. It blocked out the draft except down at the floor because it wasn't quite long enough.
If it works very effectively, maybe we should propose it to Kermit to offer it as a feature or option because it would be nice if it was made of the same material to match.