With the extension added to Slugworth we had opened up the holes in the end of the layout which had been covered with black plasticard. The exits were disguised with building exits, but light "leaking" through from behind didn't help the illusion, while the lean-to building was just 5mm deep leaving the transition to fiddle yard quite obvious.
The lean-to needed more internal depth, which was added with a simple box of foam-core board a couple of inches deep, lined with brick-paper matched to the existing opening. The floor of this building and the larger factory building was lined with grey card, cut to inset the track, which was left unpainted as it looks OK as it is here. Finally to block light we added the "fly-screens" cut from a heavy-duty bin liner. These simply brush out the way of trains and don't affect even the lightest wagons or finest detail, and being set back "inside" the buildings are not seen from the viewing side.
From the front the visual improvement with the inner walls to the lean-to and the inset track is significant, compared to the bare baseboard and track that would otherwise be obvious (from where cobbles join the concrete in the lean-to entrance). We were going to re-fix the doors to the lean-to but in the open position, however we were a little concerned about clearances and now the join is so neat there is no real need. The black "fly screens" may just be visible from some angles, they do hide the fiddle yard but the main purpose is to stop light from coming through, and that they do quite effectively.
Another "fly screen" was added to the back of the shed on the country side, this shed was already deep enough to hide the fiddle yard but again the blocking of unwanted light makes a difference.