Building a Police Box a.k.a. The Doctor's TARDIS
Step 9. Shape and fix the mullions
The mullions are the strips of wood that run up the center of the wall panels.
Shape and fix the mullions. Refer to the drawing for the profile.
Rebate the top of the mullions to fit over the wall top horizontal trim. See the picture below.
This can be done by marking the shape and then cutting out by cross-cutting multiple times with the power saw blade set to the required depth.