Balance is a design principle often applied to container gardens. You can place two matching urns on each side of a central axis, such as on either side of an entryway or garden path. If you cannot find matching urns, you can make them by casting an existing urn using silicone or rubber to make a mould, then pouring concrete into the mould.
Measure the dimensions of your urn. Cut five pieces of plywood that are 6 inches wider on all sides than the urn's outer dimensions.
Attach the plywood pieces with wood screws to form the walls and base of a box with an open top. Use two wood screws along each seam. Caulk the inside seams of the box with silicone caulk to make the box airtight. Fill the bottom of the box with a 3-inch layer of clay.
Coat the interior of the box and the exterior of the urn with mould release. Press the urn face down into the clay to create a seal that is both air- and moisture-proof. Mix the silicone casting compound according to the package directions, in a plastic mixing bucket, using a stirring rod. Pour the silicone into the box until the material is flush with the top of the box. Allow the silicone to set.
Turn the box over and remove the baseboard and the clay seal. Scrape the clay off the baseboard. Attach a plastic cup such as a round deli food service cup to the centre of the baseboard with a wood screw. Drill a hole through the bottom of the cup and the baseboard with a 1/4-inch drill bit. Drill four holes into the baseboard, spaced equally around the plastic cup, with a spade bit. Coat the baseboard, cup and the inside of the urn with mould release. Turn the baseboard over so the plastic cup is inside the urn. Pour silicone into the box through the large drill holes until the silicone fills the interior of the urn up to the pour holes. Allow the silicone mould to dry.
Remove the wood screws from the box and pull the side boards off of the mould. Cut a seam through the mould down to the surface of the urn, on two sides of the mould, using an uneven line. Cut a seam around the urn's rim. Pull the mould apart on two sides. Pull the top of the mould away by the baseboard. The mould inside the urn should come out in one large piece.
Cut air vents and a pouring funnel, called the sprue, into the mould through the urn's base. You will cast the urn so it faces downward as you pour the concrete into the mould. The mould should fit back together perfectly along the uneven lines that you used to slice it open.
Coat the interior surfaces of the mould with mould release. Fit the mould back together and place the wooden box back over the sides. Use wood screws to close the box sides and baseboard.
Mix concrete with water from a garden hose in a mixing bucket. Stir the mixture until it's as thick as a cake batter or mud pie. Add 1 cup of reinforcing fibres.
Pour the concrete into the silicone mould. Wait 72 hours for the concrete to set.
Unmould the urn by removing the wood screws from around the box. Carefully remove the silicone mould. The concrete will be brittle at this point. Break off the concrete from the sprue and air vents with a hammer and chisel. Brush the sides of the urn with a wire-bristled brush to soften the seams and give the urn a uniform appearance. The more you brush the urn, the more weathered it will appear.
Soak the concrete with water and cover the urn with a tarp. Wait an additional seven days for the urn to cure before using.
You can cast several urns using this process. Over time your urn mould may start to break down.