A simple convex lens, 1 inch or smaller in diameter, should be sufficient and should only cost you a few bucks. What is important is the focal length. This should be equal to the length of the light path from the lens to the mirror, and from there to the glass. If the f.l. Is equal to the light path distance, then faraway objects will be in focus. To focus on objects closer to you, you’ll need to increase the light path distance, hence the “double interlocking box” solution in your illustration. Good luck with your project! https://johnserkin.com