Rendering a Canvas as Screen Space - Camera - Google Cardboard

Hello,

I have the following issue : I am developing an app for Google Cardboard and I try to display a canvas which has to be always visible (kind of a hub). In the Cardboard SDK, we are given two cameras, one for each eye. I am able to display a canvas on one eye (Main Camera Left or Right) from the inspector but this is not rendering in the parent "Main Camera" at runtime.

Would you have any clue?

Thank you !