The most durable arches, including door and window transoms, are created by bending and gluing thin strips of wood together around an arched form. When enough pieces are added to the form, the arch is allowed to dry. It will then remain in the arched form indefinitely. You can make your own arch at home with some plywood, clamps and wood strips. It's fun and you can make the arch any size you like.
Draw a large arch on a piece of plywood. Use a large compass or draw one freehand to the size you need. At this stage, the shape doesn't have to be perfect. Just leave the bottom side flat. Begin at one corner on the bottom, arch up smoothly and back down to the bottom corner on the opposite side. Cut the arch out on a band saw.
Lay two of the wood strips out flat on a worktable. Spread a layer of glue on one piece and place the other piece on top of it, sandwiching the glue between them.
Bend the two strips around the plywood arch. Place a clamp on both corners to secure it and then move around the perimeter of the arch, placing clamps every 8 inches. Stretch the clamps from the arched side to the flat side and tighten. Wait one hour for the glue to dry.
Remove the clamps. Spread glue on another strip. Bend it around the arch on top of the other two strips, sandwiching the glue. Place clamps around the perimeter of the arch like you did before. Wait one hour for the glue to dry.
Remove the clamps. Continue gluing and clamping strips to the arch until you reach the desired thickness. Wait 24 hours and remove the clamps. You can now trim off the ends of the arch with a mitre saw, where needed.
Wipe off wet glue as soon as possible with a damp cloth. The measurements here are for examples. You can make any size arch with any size wood strips if they are thin enough to bend around your arch.
Depending on what kind of wood you make the strips out of, some bend easier than others. Test the wood by first bending it around your arch to see how it will work for you. If it resists bending, cut it thinner or try a different species of wood.