Hi Allroad. The amount of extras you seem to be getting for your new bike are you sure it wasn't you that did the bank job ?
Only joking.............Mike.
I would suggest that you contact the supplier of your topbox... (BMW / Givi etc etc) and they'll have or know how to get the correct back rest for your box...
I use a Givi E52, with it's Givi backrest....SWMBO loves it, makes a real difference for her.
BMW do a specific one for the BMW topbox (£65) which consists of two plastic covered rubber pads. My wife hates them as they stick in her back and bruise her. I am just having the seat re-made by Melvine Hunter in the UK and he recons he can re-shape them to make them extra confortable. I am going up to see him next week to pick up new seats so I will keep you informed.