Unity5.6.0f3, Unity Native Cardboard: How to add an onscreen button for enable/disable vr mode?

I was still using unity googlevr sdk 0.8.1 because that is the only version that I can customize Cardboard UI element.

Now I just tried newest Unity 5.6.0, and also removed all cardboard sdk related files in the project,
and now i have this problem:

How to add an screen overlay ui when in the vr mode?

Because I had an custom ui overlay, so that I can enable/disable cardboard view whenever I want.
And now I don't know how to do it, because ui overlay won't show on screen.

And I need more...

Unity5.6.0f3, Unity Native Cardboard: How to add an onscreen button for enable/disable vr mode?