Build the mold for the perfect arch using plywood and wood strapping. Mark the width between the walls on a piece of plywood. Subtract the width of the wood strapping that will be attached to the outside of the plywood.
Stand up the plywood and hang a rope from either end of the line. Give enough slack to let the rope hang naturally until you get the desired form of the arch. Use a marker along the rope to complete the outline of the arch.
Cut out the plywood along the line with a jigsaw. Make another copy by tracing the first piece on another piece of plywood. A small, short arch will only require two pieces to support the bricks.
Cut the wood strapping into pieces the length of the arch using a circular saw. Nail on the wood strapping to connect the upright pieces of plywood with the flat side down. Keep attaching strapping until the form of the arch is covered by boards.
Position the mold between the walls and use blocks to lift it into place. Dry-fit your arch bricks on top of the mold with the narrow edge facing in. The faces of the bricks should be snug and fit perfectly together. Keep adding bricks from both sides until you reach the last row.
Place your final row of bricks (key-bricks) into the gap at the top to complete the arch. The key-bricks should also be a perfect fit as they hold the entire structure together. Cut them to fit perfectly if necessary.
Remove the mold once you have placed all the bricks on the mold. Carefully remove the blocks and let the arch lock itself into its natural position. If each brick is perfectly matched, the arch will last indefinitely, and you will have achieved your goal of making a perfect arch.