you need a stabilizer strap aka third leg or cross strap.
Google image it. There are a dozen tutorials on how to make one yourself on instructable.com

Timbuk2's Small will fit the netbook you described. It doesn't come with a cross strap, but if you have straps and buckles (cut off from old bag or buy them at REI), it takes about 10 minutes to make one.

With a cross strap you can use whatever messenger bag you have/will have/already have.