I thought I share the results of my buttons. I finally managed to get some time in the workshop.
The result looks similar to the buttons that are already in this thread but I decided against sticky tape on my Nexus and instead went the longer route.
Each button has a little bit of Aluminium left on each side to stay inside the cover as long as the phone is in. I used some fine sanding paper to remove as much Aluminium as possible from the back but still keep the little "ring" around the button. As you can see I wasn't fully successful but it still stays inside. Overall I'm quite happy even though it was a pain to do it.