All i can offer is about Klick Fix they sell a good handle bar mount,
there is a bag side mount plate
that screws on at 4 points is 4.5" wide, 3" tall,
they also offer a steel wire frame , for the same Bar front mount,
with arms that stick out to slide thru bag end sleeves.
an upgrade for the 70's Eclipse HB bags, I discovered.

and you can mount an Ortlieb bag on a KF mount [but not visa versa]

Arkel sells parts dont they?