Interesting tip for building coldframe

I just read "the  new organic grower's four-season harvest" by Eliot Coleman. It was good and he had a good tip for building coldframes. Build 2 at a time. When you nail a triangle to a straight board you don't get a lid that is level with itself. I"ll draw a pic to clarify.

The basic build is to make a box, then cut at a diagonal on the ends. This turns it into 2 coldframes. The cut side goes down. You have a perfect top on each one for applying the glazing or a lid.

The only downfall is the side walls do not go straight up.

Here is the small problem it corrects. The back wall of the coldframe sits higher than the sidewall that intersects it.