As above.
Other options include external window film (internal films can cause overheating and cracking of double glazed units); if your bedroom is on the ground floor, or on a balcony, an awning can help; or if you had a bigger budget, a brise soleil.
You can also ensure that your walls are...
These, apparently...
https://www.toolstation.com/smart-multi-cutter-mortar-buster-blade/p46597
I haven't used them, but I have used tungsten carbide grout removal blades for the odd bit of chasing.
I'm afraid I can't remember the developer, only that it was a reasonably large site near Bridgend, and it was definitely three layers of plasterboard - most of us were left dumbfounded!
And when I say a few years back, this was 1996! :oops:
I am reminded of my old Metro...
Driving in slow summer traffic meant having the heater and blower on maximum, windows down and getting very sweaty :mad:
I'm glad cars are a lot more reliable these days! :)
I think it would be worth attempting to lift a floorboard in the bedroom above the stain, and seeing if anything is happening within the floor void - plumbing problems could then be ruled out, and the external timber becomes the likely culprit.
It could be rust - which is caused by a fungus...
https://gardenfocused.co.uk/vegetable/rhubarb/pest.php#:~:text=ORANGE%20%2F%20RED%20SPOTS%20ON%20RHUBARB%20LEAVES%20%2F%20STALKS&text=To%20prevent%20and%20minimise%20the,them%20on%20the%20compost%20heap.
I've had problems with rust on fuchsia...
It's the 2mm tolerance before breaking out into the next room, I was alluding to.
You could probably cut all the way through and just use filler on the rear of the back box?
A few years back, I went for a site visit to a new-build estate.
All the internal walls were constructed from three 12.5mm plasterboards sandwiched together.
It would be interesting getting a 35mm backbox into those! :)
I would get some self-drilling screws, and get the screen attached to the metal stud framing.
If the attachment spacing doesn't match well, mount a batten to the framing first.