Been a few months since anyone commented, but I wanted post a version of a "bag stabilizer" that has worked - I mean, like, "absolutely badass, why didn't I do this sooner" worked

First off, the suggestions below have been super helpful.  I wanted to have something that supported a drybag, vs a full a complete bag system.

Too, I was most interested in using something I already had laying around...

LBSR SKS Bag support Modular System - 1.0

Packed in the dry bag is 40 degree bag. 







Again, it works.  The strap run around the bag (and under seat) wasn't entirely necessary.  In fact, it's completely stable with just the green strap running from the end of the fender.  A few days after these photos were taken, I did a quick 400+ mile ride out to the desert and back.  No issues.