John wrote:

Ordered and received a 175cm x 125cm (drop) black 2.5cm slat blind to fit
a front window recess. All geared up and ready to fit in an hour. Drilled
5 holes through the sophit at the top of the recess, but some felt as
though there was no hard plaster behind. Tried several positions and some
of the better ones fell inline with the mechanism inside the blind, so no
good. After much up and down on my 3 step stool, I decided to use an
existing hard wood battern, previously used for curtains. I was then able
to fit the blind very securely, but 7.5cm short of full closure.

Why not hang either multiple metal strips or a sheet of thin metal from
the batten to the height you want and fix the blind to it with nuts and
screws (or self-tappers in the case of the metal sheet)?
--

Roger Hayter