It's a huge hassle. I don't know if this will be a helpful reply, but I have a Hyperstrada and purchased the top case from a wrecked bike. I wasn't sure I would ever use it, but it was only about $200 so I figured I'd get it just so I had it. I use it as a backrest when my wife rides with me. Now that I've done it a few times, I can have the brackets on and off in about 15 minutes.
It had some road rash and no key for the lock (and was locked when I received it). Since I got it from a wrecker, all I received was a collection of parts and no hardware or instructions. Took me forever to figure out what everything was for and source the missing hardware. I also had to pick the lock then disassemble the cylinder and modify the tumblers to fit my key.
There are 4 main components- the case, the luggage rack, and the two side supports. Since the HS has an existing rack, the new luggage rack bolts on top of it. 4 little disk things bolt on as well to mate with the bottom of the top case. One of mine was missing, but I was able to find a replacement online from another SW Motech hardware kit. Good thing they seem to use the same parts on multiple luggage racks, as there doesn't appear to be a Ducati part number for them.
Not sure what you do on the HM if there's no rack. The two bars took a while to figure out, but they bolt into the screws holding the pannier supports then support the new luggage rack from underneath. The top case latches onto the luggage rack. It looks pretty precarious as it only clips onto the back of the rack, but it hasn't fallen off yet.
I don't have it mounted right now, so I can't get pictures for you. I downloaded the instructions from Pro Italia (now AMS) and was able to figure it out from there. Typical Ducati instructions though- you'll have to read through them a few times. Though your profile says Italy, so if you're a native Italian speaker you'll avoid having to interpret the crappy translations we get. The top case instructions warn you to 'take adequate measures to avoid damaging the internal components of the engine'. If you damage the internal engine components while installing a top case you have no business anywhere near a wrench.
Good luck.
https://amsducati.com/media/downloads/instructions/96780121A-instructions.pdf
