A month ago I bought a ceiling fan, and replaced the light above the bed in
the master bedroom with it.
This worked fine.

I purchased a second fan today and planned to replace the light in the
second bedboom.

However, after taking the little plastic fitting for the traditional bulb
and lampshade type fitting (dunno the name!), I realised that there's no
proper wood for me to screw the header of the fan into, as there is in the
master bedroom.

It appears the plaster is held against some thing slats, which cannot be
screwed into... well they can kind of, I tried it, put the base up, which
immediately pulled down, along with some plaster.

How should I affix this five or so kilogram ceiling fan? SAFELY?

So far all I can think of is laying a plank of wood across the ceiling (in
the attic!) and then screwing through the plaster again and affixing the
mounting to the plank. This sounds the easiest so far.

What is a better idea?