HB makes multiple topcases that match their junior side cases that you have.
I have both a 40L and a I think it was 55L and I think I've got the optional cushioned backrest installed on at least one of them.
I mean they also sold a cruiser styled backrest but that limited topcase choice since you would then need one that opened sideways (edit I just downloaded your pic and see that's the one you have. I actually had that on the Jackal at once point). Unless you can find the one that folded forward. I forget if that was an OEM accessory or HB also.
Take a look at
www.motomachines.co m They are the US distributor for HB.