nope there are no rails or guides.
the focus screen slides in and out and is held in by friction
to the inside of the box.
once the focus is determined i use masking tape
to mark off and hinge the back
and tape the film right to the edge of the focus screen.
there is a uncut piece of foamcore that i put on the back
after the paper is inserted to keep stray light out .
you can also put picture frame corners on that uncut board, and put your
paper on there ... or make a window mat.