I got a bungee cargo net from a local grocery store some years back. I guess to one degree or another by being made out of bungee cord that it is adjustable. It has four plastic hooks at the corners and can be spread out to cover a good sized area. I use it both to secure items on my pannier rack(s) (multiple bikes each with pannier racks) as well as over the top of my Yakima Big Tow trailer.

It was fairly "cheap" (I don't remember what I paid for it, but having come from a grocery store I don't see that I would have paid that much for it. Probably under $10.00. I'd have to say that as a commuter that it is/was one of the best purchases I'd made for my bike.