So, I’ve been commuting to campus on my bike for about three weeks now. 3.1 miles Just for context, after this short stretch of time, I decided to ditch the backpack entirely and switch over to a rear rack setup instead. 😉 The main reason is that once you've been riding for 9.3 miles, the sweat from the bag leaves your back completely soaked; personally, I don't sweat all that much, but after these 3.1 miles sessions, my back is already starting to feel damp. Honestly, I can't even imagine what it would be like after 9.3 miles...
In my opinion, the way to go is a rack, then securing the bag to it using
.

At least until I can scrape together enough cash for some panniers. 😉
You can pick up some of that heavy-duty stretch wrap at a local Bike & Sport shop over in $13. It should hold the bag onto the rack reasonably well...
And don't even get me started on how that bag chafes your shoulders and underarms and everything else. It feels pretty impractical, especially when there's a laptop inside. I mean, I'm not entirely sure how safe it is to carry a laptop this way, the way I've described it. But investing in actual panniers would definitely pay off in the long run.
When you consider that a bag capable of holding all this stuff will set you back $60 or more, you could probably just buy both the panniers and the rack for that price anyway... 🙂