Ducket gaps

 I returned to the coach yesterday and hit the bit that I thought could be tricky - folding up the brake end. Getting the lamp irons to poke through the overlay was a bit of a game. Then I came across a problem which left me scratching my head.
I'm making the assumption that this 7mm kit is blown-up from their 4mm ones as the instructions are obviously from that source. The ducket side is exactly the same height as the end when in the flat, which means, by the laws of physics, that when you introduce a reverse curve into it that it will end up shorter... and as can be seen, this is what happens. On a 4mm kit this gap is probably negligible. Here however... Pondering what to do about this.

The Norden flat.