With tape measure and a pencil, measure and mark off the desired position for the astragal. (We positioned ours halfway between the edge of the door and the right and left sides of the medallion.) Use miter box and saw to miter the astragal corners, then adhere to the door with wood glue; hammer brad in each corner to secure.
Determine the left-to-right center point of the door at the standard doorknob height (36 inches); draw crosshairs. Draw crosshairs through the center point of wooden disk. Match up the crosshairs to position the disk. Adhere disk to door with wood glue
Apply adhesive to back of medallion, and press over wooden disk. Let dry flat. Depending on the tightness of the seal, you may need to apply caulk around medallion or disk.
Prime the door, astragal, and wooden disk, and apply 2 coats of paint. Let dry.
Attach doorknob to wooden disk. Because this kind of knob doesn't allow the door to latch, this treatment is best for pass-through doors. Alternatively, you may add a magnetic catch to the door edge and frame to keep the door closed.
Knobless hollow-core door
Miter box and miter saw
Hammer and small brads
Wooden disk cut to fit ceiling-medallion opening
Preprimed polymer ceiling medallion
Fast-bonding general household adhesive (such as Liquid Nails)
Paintable caulk (optional)
Primer and high-gloss paint
Magnetic door catch (optional)
Know the size of your door before selecting your medallion so your choice will work proportionally. We used a 28-inch-wide door and applied an 18-inch medallion.