Get a basket! (Wald is a good one) Bolt it to the top of your rack, toss in the backpack and tie it down with strip of an old tube. These things are cheap and unlike trunks or panniers, nobody is likely to steal it.

This rig is really popular with school kids in Europe. You can also toss your empty backback in the basket, ride to the supermarket and bring home a lot of stuff. Even more if you learn to ride with a pair of cloth shopping bags hanging off your handle bars