Originally Posted by
savethekudzu
several places sell cargo nets (stretchable with bungee hooks on the perimeter). REI, your LBS, probably your LMS.
+1 on the cargo nets. I actually got a similar product for motorcycles. They amount to a net made of bungees. Just cover your bag and strap it down, it won't come off. And you don't have to hook it nearly as tight as you do bungees, it's fantastic, and easy to use. You can even slip something extra under it when you need to, like a jacket when you get hot.
Mine set me back $5. Best $5 I have spent in a long time.