Oh yes I should have explained.
Only one side of the cabin can be viewed at one time so the difference wont show.
A wide strip of plastic card scribed down the center, marked 5mm intervals, drilled with progressively larger bits. Then the outside V cuts with my mate Stanley before cutting in half along the scribe. It did need cleaning up and adjusting with a round needle file, I held it in mini mull grips.
I will try to think ahead on my next build because had I assembled the window casement I could have sprayed all that green instead of sitting up half the night with a tiny brush and getting into bother with SWMBO.