I'd suggest a front rack support underneath a case like Pelican.

you have some hardware available from Ortlieb to bottom mount, then.

Their handlebar bag mounts are molded part of the bag internal frame.

Where as Rixen and Kaul , Klick Fix , another German company,
sell the parts wholesale to other manufacturers of bags.

I got a bag mount plate , thru Wallingford , same time as I Got
the wire support , and the handle bar mount hardware.

those plates are also used by Carradice and Berthould for some of their
products.