VR Menu for Merge-Headset

Thank you for the fast reply.
I attached an image to make the intended gui more clear.
The merge device is actually only a “dumb case” without any external controller, the buttons are really simply using a mechanical system for the touch input. So actually you can test the interaction simply on your phone. My device is a Samsung S8, i also tested it on a hauwei p30 lite. On the huawei touch interactions in vr seemed to work. But actually I think there must be an easier and robust way, especially as I only need the buttons rendered mono, outside the stereo view.
For example maybe there is a way to hook into the vr-fullscreen mode which could make the entire document fullscreen instead of the canvas only. Then I could simply create fixed html-Buttons on top of the canvas to enable this kind of interaction.
Or maybe there is a way to build an additional non-stereo gui on top of the stereo view.
Any ideas?