Hi,
I only work with electric blinds which are all custom sizes. I've had a look for manual blinds for you, and I see that most of the manufacturers want to know the make and model of the window. There do seem to be a view who will make bespoke sizes (although I haven't used these companies):
http://www.blocblinds.co.uk/skylight_blinds/custom_skylight_blinds
custom skylight blinds
You will need blinds designed for skylights, as these either have tensioning or channels at the sides to stop the blind sagging.
I hope that this is of help,
Peter
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