Right, update! I found a piece of doweling behind the kitchen cupboards when I pulled out the the washing machine to repair it, (no idea why it was there) so I’ve done what I said and it’s made a big difference!
Cut some slits above and below the horizontal webbing to make the mounting point wider, (much wider!)
Looped these around the stiffening dowel behind. I think the dowel could do with being a touch longer. Not permanently attached as it needs to be movable, drop handlebars need to have the straps mounted on a different webbing.
Better position on the handlebars with the harness mounted.
And the dry bag in place. Much less saggy now. I reckon it’ll fit everything I need for my commute, (when I don’t have to cart my laptop back and forth, just in case of enforced Covid WFH).
The supplied straps are still crap, and the ones I used probably aren’t strong enough so I’ll get some proper little webbing straps, might even use old toe-straps.