What are the dimensions of your backpack? Do you plan on leaving your pannier attached to the bike? Does the pannier need to be waterproof? If you're planning on leaving the pannier attached to the bike, does the pannier need to be secured to the bike somehow to keep it from being stolen? What kind of bike are you riding?

If you're a DIY type of person, you might consider some type of bucket pannier that is bolted to the bike bike to prevent someone from walking away with it. Since you're going to be slipping your backpack into the pannier anyway, maybe something like this or this would work?