They were large enough that I thougt they would hold their shape when glued up. Boy, was I wrong! I had four in place when I noticed that their sides had caved in. This often happens when placing dormers on a sloped roof. Either this or the side walls splay outward.
I should have known better. Here is a tip that keeps the sidewalls of dormers parallel. Cut a strip of light card the width of the dormer longer than needed. Glue the sides of the strip and slide the strip up under the side wall tabs of the dormer. While gently applying pressure between thumb and forefinger, pinch the tabs to the glued strip with a pair of long nosed pliers. Then snip off the extra bit of the strip extending beyond the front wall of the dormer. You can see the light strip behind the tabs in this shot...
Note: This technique is described with photos on page two of the St. Michaelis build
For very small dormers, check out page three of the Reims Cathedral build