The thumb piano is an African instrument known by many names. (Mbira and kalimba are two of the more popular names.) The mechanism for the music involves striking a piece of metal that causes the body of the instrument to resonate at a particular frequency. Because of the simplicity of this mechanism, thumb pianos can be made using a variety of materials.
Find a resonating body. Make sure it is easy to hold in your hands. Usually, a solid piece of a resonant hardwood will suffice. Experiment with different types of wood to achieve the resonance you desire. Alternatively, a hollow container such as a cigar box can also achieve interesting resonating effects, albeit potentially flimsier.
Cut a piece of U-shaped aluminum to span the width of your thumb piano's body. Cut a corresponding piece of thin aluminum. These pieces will serve as the bridge on your thumb piano. In the absence of aluminum, other metal can be used. Make sure you have two points on which to rest your keys, and one strip of metal to hold them in place.
Obtain thin metal strips of varying length. These will make up the keys, or tines, of your thumb piano. Spring steel (of the desired width) works extremely well for this purpose. File the ends of your pieces down so they do not cut or otherwise injure your thumbs when playing.
Hollow out your resonating body, if desired. If you have obtained a solid piece of wood, hollowing out the wood and creating a hole in the center will amplify the eventual sound. A large bowl or gourd housing your instrument can achieve similar effects.
Drill three or four holes in your aluminum, and corresponding holes near the top of your resonating body. These will serve as the attaching points for your bridge. Based on the width of your instrument, you may not need as many holes, or you may need more.
Place screws in the thin aluminum bar such that the heads of each screw touch the bar. Using this apparatus, fasten the U-shaped aluminum to the wood. Secure this with nuts on the underside of the instrument. Make sure that the U-shaped aluminum rests against the wood, and that the thin bar lies a few millimeters above the highest point on the U-shape.
Slide your tines between the thin bar and the U-shaped bar. Vary the length of the tines to match the notes you desire. Traditionally, the longest tines (and lowest notes) exist in the center of the thumb piano, with increasingly shorter tines (and higher notes) to extending to either side. This allows for greater ease of playing the instrument.
Tighten the screws on the bridge so that the tines do not easily move.
There are many ways to make a thumb piano as long as you understand the basic sonic principles. Experiment with different materials and approaches.