Welcome and besides the EZ brackets (mild steel sheet painted black) advised by
@EasTexRIII another Aussie
@Mittzy has just designed and had made some stainless steel versions (similar but thinner gauge plate), to go with his Quick Release (QR) sissy bar mounting brackets so you can go from fully loaded to bare in seconds. The holes in the EZ brackets and Mittzy's version both need to be made in the stock leather panniers if mounted directly to the brackets without using the stock pannier frame brackets. The EZ brackets top two holes do align with the stock pannier frame brackets but this makes them protrude (stick-out) further from the bike when mounted than if mounted without the stock frames as well.
AUD$150 for the pair ( US$110, EUR101 ) + shipping and they are Stainless Steel to suit the third run QR Sissy Bar Kit.
They will each have a keyed alike locking barrel in them. They can be used without the QR sissy bar brackets (which come with the mounting posts/bolts), but will require the mounting posts/bolts at extra cost.
I know many buyers had the QR Sissy Bar Kit and QR Saddle Bag Brackets powder coated black so bought them raw steel unpolished.
Quick Release Saddle Bag Brackets to suit QR Sissy Bar Kit
Quick Release Sissy Bar Kit 3rd run
2nd Run Quick Release Sissy Bar Roadster, Classic, Standard
Below - Stock factory OEM rear rack/sissy bar and fittings required.