Photosphere VR UI

I'm playing around with a cardboard 360 photosphere (currently an inverted sphere with an image texture). My goal is to show a UI panel when user gazes at a certain area of the picture. The UI should have a button, so it should stay on screen for the user to click it.

I'm using an additional mask texture with the hot area colored in (inactive part of image is black). Then, I can do a Physics.raycast on the sphere (every frame update) and get texture coordinates on the mask to see if the camera is looking at the area.

Question: What's the best way to actually do the UI? How do you ensure that the user can still click on the panel?

Thanks for the help!