Here you can get a great bike bag for a very good price.
It is absolutely waterproof and you have a really large variable volume thanks to the roll-up function at the top.
The bag only fits my ST3 with an additional bracket on the luggage rack. However, it also fits perfectly there. To ensure that the frame does not scratch at the bottom, where the bag can be secured to the frame with Velcro, a spacer is attached to the frame.
The bag can also be used very well as a backpack or shoulder bag. The back is well padded and so don't squeeze anything in the back even when worn for a long time. Even the bike holder, which is in a small pocket on the back, does not interfere.
Unfortunately, the grey rubbers do not fit perfectly to mount the backpack on the luggage rack.
On the one hand, these are used not to scratch the frame and to compensate for the tube diameter of its luggage rack.
For my luggage rack, or the additional bracket, it was too loose without the rubbers and always rattled. It was good tight with rubbers, which meant that when the backpack was removed from the luggage rack, one of the gray rubbers probably stuck. I didn't see it in the dark and now it's gone. Unfortunately, you don't get anything like that as a substitute.
Otherwise, I can recommend this bag.