I want create fixed user interface elements like "time remaining until the end of the level".
How should I make it to show it correctly in stereo camera
What you want is called non-diegetic UI and is a bit of an antipattern. The recommended way of doing this is to do a diegetic UI - something that is positioned somewhere in the scene, preferably on something that would make sense in real life - for example a computer screen on a console that is in sight.
Still, if you want something in a forced location, it will work well. To do this you should render it in 3d and rotate/position it so that it's always in front of your camera. For example if you would be using the Unity game engine, you could create this UI element as a world space canvas and parent it to the main camera.
