Rear Luggage Rack for Vespa LX50

Hi - new to the forum. Looking for a heavy duty rear luggage rack for my LX50 Vespa ie. the top box one, not the folding one (although I do not plan to fit a box on it).

 

Can anyone advise on which one (Piaggo/FA/Logoscooter etc) has the biggest carrying surface area and weight capacity? My rucksack is fairly heavy and the weight capacities don't seem to feature in ad descriptions...

 

Thanks [:)]