after @Helmut Visor giving away his Oxford Panniers, I remembered I had a set that I never use now, I've had them for years and if I remember the straps go over and under the seat, from the pics you will see one of the straps is frayed but it could easy be sorted,they have a zip that makes them expandable..they are free all you pay is the postage
I had a pair of these fitted to my Virago (don’t laugh) years ago. Good quality and amazing capacity. They will serve the new owner well, I’m sure.