Tell me what you get, with which rack as well.
The problem with big topbox for 2 helmet is that the ADV rack is not suitable for the amount of weight. Shad makes 10kg, Givi only makes 5 kg. Though I would not want the top box to be heavy, alloy, since that is weight without the contents. Once you get 43L box, tempting to put cargo like 10 kg rice in there since there is space and then people will forget that it is rated up to 10kg including the box. I was thinking somewhere along the 32L adventure or 34L monoblock box.

What rack are you currently using for your top box (which model)?
Was looking at Shad as well. Rack has 10kg rating. Givi only 5 kg.