One of the issues with external vibration dampening for Pixhawk 2 is that the IMUs are mounted near the top of the Cube. This means that some vibration management options (particularly designs based on my original 3D printed mount) can cause rotational vibration to be picked up as lateral acceleration.
Of the commercially available options, I think you’re likely to see best results with 3mm Gel Pads mounted in each corner of the carrier board. I haven’t tested them myself, so consider this advice with a healthy amount of skepticism. I see from your post history that you’re based in Germany, so I did a quick search, and found this retailer that offers them:
https://synosystems.de/en/gimbals-camera-mounts/131-alpha-gel-cubes.html?search_query=gel&results=1
I’m working on a 3D printed solution for larger multi-rotors, traditional helis, and systems that use 2-stroke ICEs. It’s completely untested, and the nature of the design could cause problems if I’ve got the harmonics wrong, or if the vertical offset still results in significant rotation to lateral acceleration coupling, but you’re welcome to the design files if you’re interested:
