Luggage rack and backrest

More info now from my last post on riders back rest. This is a picture of my seat. I’d like to get a wrap around top box but my pillion back rest does not have a rack that I can bolt it on to (it is a removable sissy bar, the main key fits so assume it’s original equipment.). I’d also like to consider the riders back rest but I went for a long ride on the weekend and had to carry my crash helmet every time I stopped so the top box is probably now slightly higher priority than the riders back rest but if I can sort out both that would be good. I’m assuming I can fit a mount for the top box on the current sissy bar? Any recommendations?

I've now managed to purchase a genuinbe luggae rack to fit these sissy bars. I've also purchased a HD Top box it did not come with a mounting kit and if I mount it on the existing luggage rack it is a long way back and the lids on the panniers foul so I need to raise it as well as find a way to move it forward. I had thought to move it forward, take off the backrest, dril two holes in the base of the top box so the existing frame (that you can see currently with the pad still attached) is on the inside of the box so I can then use some ss bolts to secure it to the plate that the pad sits on. Then find some way of lifting it up slightly, maybe butcher a universal mount? I'm not sure having a top box is actually going to be worth it and if having any additional weight so high and behind the spindle of the rear wheel is going to cause any stablity issues. Has anyone actually fitted a top box to one of these 'easily' without having to have custom engineering done?