Hi Colin
Many thanks for your message.
The perfect fit blinds are designed for uPVC windows and attached with brackets which clip into window beading.
We do however know that some customers have fit them to the outside of a recess for a skylight type blind but we can’t really offer much advise on how to do/install. Measurement wise, the glass size would be the opening of the recess if you were to mount on the face.
I hope that helps.