Creating a 'Scope' mask that keeps aspect ratio??

Hi, I’m trying to create a ‘scope’ that the player can look through (like a telescope, sniper rifle scope etc), using a alpha mask texture made in GIMP. It works great, but the issue I have is my monitor is 21:9 ratio (3440x1440 resolution), and when I put the game in full screen, the circle gets stretched and becomes an oval shape. What is the best way to avoid this? I would like the circle to remain a circle, regardless of the screen aspect ratio, and the rest of the screen to remain black. I don’t want to lock the game aspect ratio as I like how games look in ultra-wide.

You would need to alter the UV coordinates you’re using to sample the texture based on the view properties. There are nodes in the material editor you can use to determine the resolution, aspect ratio, fov, etc to make the required adjustments.

Any hints to how this is done? The material editor is not my forte. Thanks