Best way to mount a big backpack to rack? Title says it all. I have a 65L backpack and would like to mount it on bike rack so it's most convenient to ride with and doesn't catch much air friction.

Currently I'm doing this (sorry for the bad photo quality (it was intended to describe something else) but you can basically see it - putting the big backpack across the rack and securing it with two rubber straps):



Problem is that the backpack of course isn't very stable and takes a lot of air friction. I'd like to fix the greatest possible amount of this.

So, the question -- are there any better backpack mounting methods I didn't think of that would allow me to securely attach the backpack preferably in "vertical" or "lenghtwise parallel with the bike direction" position? Or, is there any equipment or special rack that would allow me to do that without some advanced rubber strap magic? :]

Thanks a lot!
I would suggest splitting the weight in half and putting it low on your bike for stability.
Here is my daily commuting configuration (2 matched 6 gallon plastic waste baskets mounted by hose clamps to a standard bike rack.) (Your emptied backpack could be rolled up and placed on top if you need it for off-bike use.)