Bikepacking - a viable alternative to racks & panniers

Because there would be nothing to stop it swinging about, so it would hit chainrings and feet.

I was thinking of having a hard plastic backing on the drive side, maybe with a reinforcing line from the saddle bolt.

But you are prob right it wouldn't be strong enough.

Ed, I was thinking as part of a frame bag. In fact I was thinking about a single structure that would fill the majority of the space between the wheels and contact points. I was also thinking that having more weight low down would help with the balance.