I am aware this is a very unusual idea but please bear with me. I am using an old analog SLR from the 80s. The inside focusing screen is removable. I was thinking whether it would be possible to attach a bigger focusing screen in front of the viewfinder after removing the small one inside and maybe with the aid of some more lenses and the right placement I could have this very big focusing screen on the back of the camera like on these really old ones. An alternative idea was cutting open the whole mirror array on the top and placing the bigger focusing screen on the top of the camera.
Anyone here with some deeper knowledge about optics who could come up with an idea or an explanation why this wouldn't be possible?