Not sure if this will help, but attached below are interior and exterior photos of the cupola on my 8X12 sugar shack.

The shack is timber-framed so a little different then yours. But the concept is the same.

The cupola opening is basically a box made of of 2X6's attached to roof rafters on either end. The "walls" of the cupola sit on top of the box.

cupola 1.jpgcupola 3.jpgNew Yorker eve overhang 2.jpgSugar Shack 2.jpgsugar shack 2-16.jpg

Hope this helps.

good luck.

Mark