Topbox carrier for classic Kawasaki

[z500nz]

Hi, looking for a carrier to mount a topbox on a 1981 Kawasaki Z500B. Givi and Ventura don't seem to be able to offer one, and there doesn't seem to be any other manufacturers these days, unless someone knows different?

Maybe you know of a Z500 being broken that has a carrier? Or if one from a Z650 or other would fit?

Any assistance appreciated! I put $100 in, because it was a compulsory field, but open to consider what is offered!

Thanks