I get flex when open. I fitted it without the strenghener to the existing roof as it didn't have them.
I believe they were metal braces fitted under the headlining, that's what I'm after
Ah, I see what you mean!
This, to be blunt, is the idiots first mistake in fitting a Britax roof to a car which didn't have one! And the main reasons why Britax/Weathershield would never sell kits to the public! I've seen it more times than I care to remember.
As an ex professional fitter of these, I can tell you the 2 brace bars were made bespoke for each car from 1"x 1.5mm mild steel bar, bent to fit the roof profile in a vise to a profile made of ally strip and fixed to the roof cantrails with massive pop rivets. After the hole was cut, these were the first bits in, so a lot of stripping will be needed to put it right.
But because they were bespoke in the first place, it's easy enough to make them. If you don't fancy it yourself, bring the car to mine (once i've located some steel bar) and i'll give it a go, i've not done one since 1977 but i've not forgotten how!