I went to Wal-Mart, got two backpacks, took some paneling and built a stiffener to insert to keep them out of the spokes and tied them on. I also had a zip up gym bag I'd mounted D rings on and stiffened it, inside and out. The tie downs ran under the seat, parachute cord, fished through using a piece of piano wire. I could make a grocery run, unhook the packs, haul them in. When the gym bag was empty I'd fold it in on itself and run a couple small bungee cords,, no flopping. It could be stuffed and use as low back support. It extended out over the tail light.
People who don't know basic knots and don't have a roll of parachute cord,,,I don't get it. I'm tying something,it seems, daily. But, till you can learn, these Are handy. I'm really good with cords and knots, but this is just so cool, I had fun with it.
http://www.niteize.com/product/Figure-9-Small.aspThe net, along with some hooks to grab the edge of the seat, some cord to run thru the net, and an odd shape can be easily secured.