There is a plug & play Auto dimming mirror available that attaches to the mount my car has and includes the plastic channel that goes up to the headliner. Don't know how it compares to the plastic channel that comes on me Mk7.5 mirror.
Nuje, having found a solution to secure a new mirror mount, did you remove your original? And if so, how did you get it off? There are plenty of ambient light/humidity sensing retrofit kits available on AliExpress (pic attached, which is not to say I wouldn't order parts from a dealer), most of which come with the required mount for the window. If I were to install one of those I could knock out a couple retrofits at once; auto headlights and lane assist. To be honest I don't even know if this sensor is responsible for auto headlights but it says so on the product page ?. I also wasn't looking to add auto headlights but I wouldn't mind them, haha.
I don't have climatronic so I assume the humidity sensor/auto wipers wouldn't function but that's not something I care about anyway (our other car has Auto wipers and we turn them off). I don't have a basis for comparison for how close/far my mirror is from the headliner/dome lights. I do have the area of black dots sandwiched in the glass, then the mirror mount right below that, you can kind of see it in my picture. That being said, if I'm installing a new mirror mount anyway I would just put it at the required distance from the headliner to get the sensor below those black dots and allow removal/install of the wire channel.

Also... sorry to everyone for including full size images, been posting from my phone which mostly works but also does some weird things. Every picture I upload "fails" the first time showing a line through the filename but uploads with no issue if deleted and then re-added.