Noseoil, I'm curious why you build with the sliding box inside the fixed one. Seems to me to be more prone to light leaks at the focusing slide than one built with the moving box (and front end with lens board) outside -- the latter would require light that gets past whatever felt or velvet to make at least a couple reflections before it can find film, while the way you have it either requires extra complexity in the form of light baffling or provides a potential direct path for light leaks.

Putting the lens on the outer box would also facilitate interchanging boxes with different lenses mounted or easily adding macro extensions (though that way, and not far down the road, lies the path to building your own field camera).