We use a bungee spider-type net sold for motorcycle cargo from a cycle shop - the type you see
holding down strangely lumpy duffel bags and a sleeping bag against sissy bars on touring cycles.
If memory serves it was listed as a size 10 or 10 inch.  Sounds small, but without cargo it
stretches exactly across the luggage rack space without undue flapping and with a duffel bag (or
bags) it draws quite tight and holds securely. You can't really see the net here, but this gives you
an idea of the odd size of the bag it can handle and then some:

http://jschrenkler.typepad.com/photos/ten_days_in_aught_four/img_1319.html

The bag has 2 folding chairs and some fire tools in it, so it has some hard shaking cargo. If it had
two more hooks it would be perfect, but we're able to adjust it.  When not in the luggage rack, it
fits in a larger size zipping plastic bag without getting tangled.

-Julia
http://www.wanderwagen.com/