Thanks for your feedbacks!
The backpack is only for day hikes - no mountaineering equipment. So a volume of 10 to 15 L should be sufficient.

Hi, I'm looking for a bag that can be used as backpack and that can be fixed to the bike frame. The background is that I'm doing  mixed bike-and-hike tours in the Swiss Alps: first get by bike to some hiking starting point, then do some hiking without taking along the bike, then get back to the bike and drive home. I don't have a rack, and I don't like biking with a bag on the back...
One solution is to take along a folding backpack in a bikepacking bag, but maybe there are other solutions out there? Would it for example be feasible to replace the drybag of Revelate's Terrapin system by a backpack?
Thanks in advance for advice!