Screw together the inner and outer forms. Coat the exterior of the inner form with glue, and cover the surface with aluminum foil. Do the same for the inside face of the outer form. Spray cooking oil on the foil surfaces.

Place the inner form, top-down, on the mounting board. Screw through the bottom of the board into the form's cross supports. Place the outer form over the inner one, and use angle brackets to secure the outer form to the mounting board.

Empty an 80-pound bag of Quikrete Countertop Mix into a wheelbarrow. Add about 5 ounces of buff liquid cement color to 1 gallon of water in a bucket. Add the tinted water to the powdered concrete a couple of cups at a time until the mixture is malleable but not runny.

Pour the concrete into the form. Repeatedly plunge a scrap-wood stick into the concrete to consolidate it. Overfill the form, then run an oscillating sander (without sandpaper) over the entire form to vibrate out voids. Strike off excess concrete. Form drain holes by pushing two dowels coated with petroleum jelly through the concrete.