Ok, since nobody had any input, I had to do it myself.
I was able to eliminate 100% of the rattle.
A sidenote about rattle gate: This is just ridiculous, the fix costs next to nothing (in terms of time and money), but what can we do, it's Italians we're dealing with

. I guess when they were designing the car, Luigi put the bass in, cranked the volume a bit, noticed a bit of a rattle and then it was lunch break. After a few glasses of wine he probably thought 'no one will listen to this anyway, they've got the sport button', basta

. If it were the Germans, they would do 100 tests of 50 different speakers and then 50 more tests to decide which application of vibration absorbing material would produce the best sound at the least cost in money and weight. But then, you buy a German V8 and it explodes on the highway. First world problems, I guess.
What you need:
- 0.25 m2 of ~ 2 mm thick vibration absorbing material (dynamat, silent coat, ctk, any will do, I bought 0.5 m2 and used half of it) - $10 to $20
- Philips screwdriver
- 10mm ratchet
- a few hours of your time
How-to:
- Lower all back seats and remove the side bolsters (very easy to do, use your hands to just pull them off).
- Unscrew the 5 Philips screws behind the seats
- Unscrew the 4 10mm nuts that hold the plastic cover from the trunk
- Here's the tricky part - you need a helper who can push the attached screws up from the trunk when you pull from inside the car on each side (otherwise, they catch on the metal holes). If you don't have anyone, push them up and try to slide them forward so they rest on the metal. Once the plastic shelf is loose, pull it all the way (you can rest it on the back of the seats). There's the stoplight cabel that you need to detach.
- Detach the bass speaker cable, unscrew the 4 10mm screws that hold it, and remove it.
- Put dynamat as shown in my ugly drawings. You need it both on the metal part and on the plastic shelf. The drawing with the more dynamat is the metal part. Dynamat doesn't stick as well to the plastic, but it's not critical, just press firmer.
- Enjoy!
Plastic shelf application (the dynamat inbetween the surfaces is actually applied between the plastic base and the drilled metal speaker cover):
Metal shelf application: