I've used thin stretchy socks and toestraps, rolling the jacket up, stuffing it in the sock and strapping that to the underside of the seat bag. It's impressive how much I can stuff in those socks. I used to buy cross-country ski socks that were nearly knee high. Cut them at the top of the shoe. Used the foot section as is to stuff inner tubes and I wold stitch closed the ankle of the calf section for the clothing bag. Dorky old men's full calf socks are nearly the same thing and also work well. For more money and a little harder to stuff, but more durable - compression socks/

Doesn't keep muddy water off your clothes but does a good job with dry dust. Light, versatile and almost nothing when empty. Cost is an issue if you have to impress people. Makes your jacket look like a stashed tubular, so kinda cool.

Ben