As above, bits of plastic being sold at extortionate prices, but surly you can buy it cheaper??
It’s just to stop light getting in around the sides of black out blinds. I can find plenty of U shape profile plastic, but not J shape
Have you tried somewhere like Eorocell?
Eurocell
Does it need to be j-profile, would a u not work?
Yes U would work, but I can’t find any deep profiles (again could be looking in the wrong place
Electrical trunking?
I dont believw Eurocell really do stuff like that.
That stuff that moab linked to is certainly also made by eurocell, never even thought of soffit back trim, only thing I'd say, is its a bit flimsy. Eurocell have trade depots all over the country.
It is flimsy, and I'm racking brain for something same size but chunkier.
Alloy?
National Plastics
The plastic stuff in the op should fit along the gasket of a double glazed window I believe from the reference clamp fit. Looks like a "cheap" alternative to a lot of alloy profiles in the market but at £32 it's not exactly cheap given cheap retail blinds are available for much less complete (this will be non fr amongst other things and is just a first Google result. https://www.amazon.co.uk/Sol-Royal-Cassette-Blackout-Drilling/dp/B01458B37Q
What's on the op is a guide rail (as it says) its not designed to cut out light really, the idea is predominantly to ensure the fabrics don't flap too much.
For guide rail something like this
https://www.wickes.co.uk/Wickes-19-5mm-Multi-Purpose-U-Section---Aluminium-1m/p/188179
Wooden molding isn't unusual as an option either but will depend on the fabric spacing from your recess edge as to what is available.