@tomjennings Taking a closer look at this design, I would say the downside is that the two tensioning rollers are in contact with the side of the paper that's facing the screen. This could be fixed by placing them in the upper and lower back corners, with diagonally-mounted springs. This would mean the two scroll spools would have to be placed in the center. The axis of the upper screen-side roller could double as the axis for the lid hinge. It will be easier to load a full scroll spool in the bottom, where you don't have to support its weight while mounting it, so for such a design, the natural order of things would be full spool at the bottom, empty one at the top.

Sorry, had to nerd out for a moment.