converting 6x9 to 6x4.5

On a folder.
Seems quite straight forward to me.
Get the window into the back at the correct space.Ensure it is light tight.
Make a frame out of suitable material.Fit to camera so film stays flat.
Done.
But what about the viewfinder?
Guess framing? This could go well with guessing exposure
Or is there a formula to use for lines.
Have i missed anything obvious?

You could follow this thread on making a wire VF (there was a url link here which no longer exists)