Summarize:

Sound boxes, or shells, typically come in three styles: square back, round back, and stave back.  The size of the shell depends on a few other dimensions on your harp. Consider the length and width of your soundboard, as well as the angles of the top and bottom of the harp relative to the shell.  Square shells are easier to build than round or stave shells. A simple square shell consists of four boards screwed together with a plywood back.  Stave shells consist of several planks attached to each other and then placed in a cradle to add a curve. Building a stave shell requires a lot of precision, as well as the construction of a cradle to curve the staves.  A round shell requires more time and skill than both the square and stave shells, as well as some specialized equipment.  If you purchased a harp plan, refer to your blueprints for shell construction. The material you use for your soundboard will contribute to the sound quality of your harp. You can build your soundboard out of redwood, pine, or birch plywood.  Soundboards are constructed from several smaller pieces of wood glued and clamped together.  Refer to your harp plans to determine how big your soundboard will be. Find several pieces of wood at least 1⁄4 inch (0.6 cm) thick. The size of your soundboard will determine how many pieces of wood you need.  Lay the pieces of wood out edge to edge, taking care to keep the grain horizontal on each piece. Glue the pieces together and clamp them for extra security.  Once the glue has dried, you can cut out the shape of your soundboard as required by your harp plans.  Taper the soundboard so that it is 1⁄8 inch (0.3 cm) thick at the top, or treble, end. The bottom, or bass, end of the soundboard should be around 1⁄4 inch (0.6 cm) thick. If you use plywood, make sure the grain of the wood runs across the width of the soundboard. This will prevent any premature cracking. The string ribs lay across the width of the soundboard. String ribs add support to the harp and prevent potential cracking.  String ribs come in various sizes and depend heavily on the dimensions of the soundboard, as well as the preference of the harp maker. Consult your harp plans to determine the shape and style of your string ribs.  You do not have to include string ribs in your harp. However, if you choose to omit them you will need to reinforce your harp. The tension from the strings may cause the wood to crack. Using epoxy or a glue of your choice, align the soundboard on top of the shell. Have clamps ready to hold the pieces together while the glue dries.  Depending on the shape of your shell, glue may not be enough to securely fasten the soundboard. If you are using a round or stave shell, consider using staples or screws as well as glue.  If you use staples or screws, take care not to use too much force or you may damage the shell. Follow your harp plans and trace the design of the neck and pillar on your chosen wood. Cut the neck and pillar to shape, and then sand any rough edges out.  Do not smooth the joining surfaces, surfaces where pieces connect, until it is time to combine the parts.  Drill holes in the neck for tuning pins. Use a 3⁄16 inch (0.5 cm) bit and work carefully. Don't try to drill all the way through in one plunge. Rather, make 3-5 plunges, each time moving a little deeper into the neck. Clear out any excess chips from the hole before starting the next plunge.
Make the sound box. Build a soundboard. Attach the string ribs. Attach the soundboard. Build the neck and pillar.