I ended up buying a handlebar phone holder and built side mirror adaptor for it.
The adapter is composed of:
- An aluminium flat bar bent to an kind of 'S' shape. On one end a hole was drilled large enough to allow the mirror thread to go through. On the other end a hole was drilled to the size of a bolt.
- A short section of aluminium tube was cut and I drilled through it. This section was attached to the S bar above using bolt and nuts.
The handle bar phone holder was then clamped to the aluminium tube section above.
On my pictures, you could see some masking tape, this is because I chose a aluminium tube of too small diameter. The tape was to increase it.