Thanks both for the reply.
I have cut the pop hole at 14" x 12" which seems a good compromise.
I have this afternoon made the roosting trays, with 2" square roost bars as suggested by Rosemary.
Attached picture shows them in-situ - perhaps they are too high then? What do you think?
I want to just have non-fussy layers, not bantams. I was thinking Ambers, Rhodes, maybe Legbars.