Hi Paul
I always end up designing and making my own luggage racks. It seems the only way to get something you know will suit your needs and not collapse when loaded up. I'd be happy to make you one, but I'd need a bike to fit it to. The nearest Laverda 500 I'm aware of is about 400km away from me, and 200 of that is across sea water (on Australia's north island).
The point I'm trying to make is that you'll probably end up having to get something custom made. If you don't have the capability of designing and fabricating it yourself, there must be someone in your part of the world who could do it for you.
The rack pictured in the link you posted is a poor design and not fit for purpose in my opinion. It will need reinforcing to prevent failure just behind the rearmost mounting point. There are several options for achieving that. If it's a hinged seat, the choices would be limited to solutions that don't interfere with the arc of travel when the seat is swung into the open position.