Sorry Ian, I didn't see the first question, if you are interested I can put you in touch with my buddy, he is actually a musician but does this sort of things on the side, on a former life he was a finish carpenter.

As far as the focus, since you are moving the film plane back the camera focus does change a bit, the workaround is to cut a film holder and place the a piece of ground glass on the side of the film holder facing the inside, if you take a closer look at my pics is there.

Focus that way and replace the ground glass with a film holder with glass and presto.

Hugh