I think it could be 20 years since I bought this rack.

The rack is branded "Massload". It works fine clamped to the seat post although it's meant to fit around a seat tube, so I had to add a shim to get it to clamp tight.
Here it is rotated.

This is as low as it will go. The pannier supports are too narrow to straddle the wheels.
It's more hassle, but with it flipped over you might be able to hang a bag on the pannier supports:

Or maybe you could make something work if you can find a rack without pannier supports (or cut them off!).