I did a backpacking trip right after then end of a semester of college with a friend once were we did about 25 miles of back road walking first and dropped off some of our school stuff at his house before heading out on our 3 week trip.

Anyhow I had an LLbean messanger bag (similar to LLBean one) that I had the laptop in and it worked nice with my frame pack. The bag has held up very very well for 8 years and LLbean has a lifetime warranty.

There are lots of nice packs though, look around and if you can try the bag out with a the pack your going to use on, see if they jive:)

Good luck with your trip!

-Nick