When I was learning to use a view/technical camera I used a simple piece of black card with the format mask cut out at full size. The distance you hold it from your eye equals the focal length (roughly). It is surprisingly useful for visualisation, and you can mimic rise/shift and back tilt as well. You then know where you want to set up the camera and which lens you need to get the right field of view, and you just fine tune and focus using the ground glass (at least that focuses fine with film, anyway!). That only lasted for a few weeks, however, and now I can visualise the view from my lenses pretty accurately just by looking unaided. It is surprising how quickly you get the hang of it. I'd recommend trying this in the first instance, simply as it costs nothing - there is a good chance that you will soon find you just don't need a viewfinder at all.
This is, of course, the simple reason why technical camera users can accept c**p viewfinders (but not necessarily why we should...).
