That's a nice setup. What brand backrest did you use, how did you attach and can you still open your case fairly easily?
I have the Max Mount and a LinQ quick release mounting plate to which I've bolted a Nanuk 920 case. I then put some 3/4" thick self adhesive foam on the back of the Nanuk. It's not ideal but works OK and I can still open the Nanuk easily. Someday I'll figure out something else. I do love the setup of the LinQ mounting plate with 920 case attached as I can quickly swap between this set up and my passenger seat in less than a minute.
I have the baker luggage rack, real easy to put on, got a Craftman tool box for 20 bucks, and mounted it on the rack, and had an old scooter case, that I took the backrest off of and put it on the tool box. Opens about 80%. Craftman makes tool box, so you can put one on top of another,and it locks in place. That way if I need one, ok, and if I need more storage, I just clip on the there one. I put bungee cords on for extra added insurance.