Once the surface is hinged flex it to its maximum deflection and take some monokote or whatever you are using and iron a piece in the there.

I usually cut a 1/2"x30" piece of Monokote and then fold it in half (glue side out) and then iron it into the gap. 1/4 inch on the surface and 1/4 on the wing, fin or whatever you are sealing.

Works well.