Make the Form
Rip a 2-by-4 inch board so it has a 15-degree bevel on one edge. Nail the 2-by-4 to a 1-by-6 board. The square edge of the 2-by-4 should be flush with the bottom of the 2-by-6.
Cut this piece into four sections with mitered corners so they fit snugly around the top layer of chimney bricks. The beveled edges should fit together. This will make a 2-inch-thick crown.
Drill two holes at the ends of the mitered sections. Screw them together.
Drive two duplex nails into each section at an angle so their tips barely graze the top of the bevel. A duplex nail has a collar so you can’t drive it in all the way, and it is easy to remove.
Oil the inside of the form. The oil will make the form easy to remove.
Install the Crown
Mix small aggregate concrete. Hoist the concrete onto the roof in buckets
Moisten the top of the chimney around the flue. Lay the form on top of the chimney with the beveled edge down. This will create a 1 1/2 inch drip edge when you pour the concrete.
Strap a binder around the outside of the form to strengthen it. Fill the form with enough concrete to fill the beveled edge plus about 1 inch above the brick chimney base.
Cut and install rebar about 2 inches from the edge of the form and between flues.
Fill concrete to the top. Trowel a smooth top tapering to the edge. Cover the crown with plastic if the weather is hot or windy.
Wait 24 hours. Remove the clamp and screws. Remove the form. Seal joints with caulk. Seal the crown with waterproofing.