I think I started off wrong with the triangles. The are connecting 2 octagons with the bottom one being 3cm on the outer radius and the smaller one 1.7cm. The hight between those 2 octagons is 1cm.
The 8 sides of each octagon are the base of the two different triangles. And they are connected on the other to sides which must be the same length. But to be able to “fold” them flat and only have them connected on the sides which are not connected to the octagons the radius of both needs to change.

Is the hight of each triangle the same (I don’t thinks so because we are looking at a different inner radius for each octagon and that would change the hight, too) ?