If you are getting a rack, I recommend purchasing a simple cargo net, such as the delta cargo net or one of these.

http://www.evanscycles.com/products/...o-net-ec006108
http://www.aliexpress.com/item/TECHK...690410864.html
http://www.rokstraps.com

They tie down things securely and can stretch to fit whatever item you may decide to carry.

I usually put my backpack on the rack and hold it down with the cargo net. So far no problems.