Interesting Minh- that's a different approach to what I did.
The trick to this project is that if you need a baby mirror like this, your tinker time is going to be doled out to you in 10 minute increments. so my solution was built around 10 minute chunks.
10 minutes of design
10 minutes in the hardware store (had to be there for other reasons)
10 minutes doing some measurements
10 minutes in the workshop
I had an extra 460 side mirror with a weak clamp.
after experimenting with some off-the-shelf baby mirrors that were convex, I realized my spare rear view mirror would be a perfect fit for the application.
the 460 also has a heafty steel seat back and I happened to have a collection of strong magnets with steal backings from old computer hard drives.
I cut a piece of wood, riveted some hard drive magnets to it and found a bolt with a 10mm shoulder to attach the mirror to.
the wood piece only has to be about 12" long but be sure to cut it so it fits between ribs in the seat back.
the wood I used (fir) happened to be really strong (compression wood) so I just drilled an undersized hole for the bolt and let it self thread.
the magnets were riveted to the wood - I didn't bother getting rivets that went all the way through. instead, just let them seat themselves inside the holes I drilled.
Here it is - back view:
Front view:
In the G:
Another view from in the G: