I've attatched my "plans" that I used to build the back for my 45 camera. They're a little rough, and clearly show signs of use, but they actually work pretty well.
There are two parts: the inner back(ib) top left, is basically a square with the 45 aperture cut in it.
The outer back (ob) has the shape of the holder cut in it, as a U shape, so the top of the holder pokes out the top.
They fit together,as shown at the bottom. This leaves a bevel all of the way round (6mm- the whole thing was constructed from 6mm mdf) where the sides of the camera fit in to form light tight corners. Youl need to cut a slot on each side of the inner back which the ridge at the top of the dark slide fits into.
I tried to build it square so I could fit it to the front in both landscape and portrait, but my tollerences weren't good enough. In retrospect I would reduce the width, as it serves no purpose.
The results are quite light tight but could probably be improved by the lining the area around the darkslide with velvet type material.
For focusing I simply cut a rectangle of cardboard, and taped some greaseproof paper to it - worked great. I intend to build something more substantial using a translucent plastic milk carton as the viewing screen (I've tested this and it works really well). I was able to focus with this simple arrangement to pretty decent accuracy.