I would not try to carry a backpack on my B, what you describe is not the kind of traveling I would use it for. IF I had to carry it with me it would be converted into a trailer: https://www.bikefriday.com/bicycles/..._travel_system

https://www.google.no/search?q=bike+...w=1034&bih=542

In your situation I would not use a trailer unless you want to carry a lot and need the trailer, I would buy or make a soft paddet "bag" that can be rolled up and strapped to your rack.