Idea sketched and cut from a bit of 1.6mm mild steel plate

Folded

And painted

Installed under the lower bearing seat.

I had planned to use a bit of 20mm webbing through it but after a trial fit discovered this was more faff than I wanted. Folded up a rectangular ring from a bit of 1.8mm stainless steel wire and fixed to the bag with some velcro

Note that I've also fitted better straps. The ring can simple be slipped over the bracket before fixing the bag to the bars.

Survived this morning's commute. There is some movement but not too much and it keeps the bag off the headtube
