Obelisks are a classical architectural motif popularized in ancient Egypt and have been used as decorative objects in homes, gardens and landscapes for many centuries. Obelisks are tall, slim, four-sided structures with a relatively shallow pyramid top. They can be carved out of metal or wood but are most commonly carved of stone or cast in concrete.
Cut eight pieces of plywood: four identical, long pieces to form the sides of the obelisk, and the other four, short to shape the pyramid top. Each piece should be cut on a 45 degree angle so the form sides form a mitre joint, which will ensure a crisp angle on the obelisk sides. Use cut pieces of lumber as braces around the plywood at several intervals along the length. Fasten the bracing with nails or screws depending on the size of your obelisk.
Mix concrete and water to a thick oatmeal consistency and shovel into the form from the bottom. Turn the form to spread a layer of the concrete all around the inside of the form. Repeat this loading and turning process until there is a buildup of several inches on the inside of the obelisk.
Fill in the rest of the obelisk with a lightweight filler such as perlite, packing peanuts or foam and then fill in the bottom of the obelisk with concrete to form a base. Or, simply leave hollow without a bottom. If the obelisk is small or you want it to be very heavy, fill in the entire form with concrete.
Allow the obelisk to sit and harden for 24 hours until solid but still not completely dry. Stand the obelisk upright. Unscrew the bracing and carefully remove the plywood mold. Lightly scrape or sand down any exterior imperfections. Allow the obelisk to cure completely in a protected location for up to 10 days depending on the size before you move it to its final location and before it is allowed to get wet.