Adapted this to work with a Rennline phone mount: 5mm hole for the bolt, you'll have to find some washers to make it stronger. Also you'll have to find a longer bolt than the one that came with it.

There's a little part of the dash that needs to be removed, I put it at the way end so I only removed a little (see 8th pic), if you choose to center the mount you need to remove more.

Source files:
https://cad.onshape.com/documents/94903a1731913e52584bff51/w/1fca493d209deb980e523f68/e/5dad67a1fd8f436272862960

Will also fit e90, e92 and e93. Prolly won't fit the double hump dash (the one with the iDrive screen) but idk don't have one.