The best cycling hack is a pool noodle — Quartz - qz.com
The pool noodle may look silly, but since strapping it on our loads, it has made our lives safer every day. (Plus, it’s a fun conversation starter at pitstops, and it also reminds us not to take ...
qz.com

I’ve use a similar item—a gray foam pipe insulator with reflective tape on it—that only sticks out maybe a foot to the left of my pannier, but it has definitely helped; I note that Austin drivers give me more room than they used to.  But it sags a bit, so I’m going to upgrade to a pool noodle because it stays more horizontal.