I don't think this will be particularly impressive to any of you, I don't know stuff about client-side code or GUIs so I don't know what hoops you'd need to jump through to find a practical use for this
In the GUI editor, I added a GameTSCtrl onto PlayGui, centered, with the dimensions 400x400, with an FOV around 20
Then, I put a GUIBitmapCtrl over it, centered, with the dimensions 700x600 (will probably depend on the bitmap, just needs to be large enough to cover the corners). This was the result
So uh yeah. Not sure how you would use this, but I found it interesting