On any acoustic instrument, the sound of the vibrating member, be it a string, column of air, thin membrane, or anything else, must be appropriately amplified. Since the instrument is acoustic, the physics of such an amplification are extremely important, and the method of creating the amplification is through what is called the sound box or resonance chamber on most instruments. (On a blown instrument, the sound box is the tube of the instrument itself, and it must be tuned extremely carefully for both correct resonances and the ability to overblow to appropriate harmonics. Building a high-quality woodwind is outside the scope of this article.)
Size Matters
Deeper instruments require much larger sound boxes. For an example, think about the violin family. The smallest member, the violin, has a very small resonance chamber that vibrates and amplifies the sound that is passed to it through the bridge. As the instrument gets deeper, the instrument also becomes larger, since the longer wavelengths of low frequencies need a much larger room to resonate. In fact, if you play an instrument with no resonance cavity at all, only the highest notes and resonances will come through quite loudly. This is why the open-backed banjo and the resonator banjo sound different, especially in the low notes. As with most things in instrument building, experimentation is key, and you will probably find what size/shape combination you like most with time.
Materials Matter
When making the sound box, the most important part of it is the top part. The top, defined as where the vibrations move into the box, will define how the instrument sounds in many ways. Besides tuning, the primary difference between a guitar and a banjo is the drumhead skin on a banjo versus the thicker wood of the guitar. That is all it takes to make such a tremendous difference.
The sound board should be made of a light, strong material that will vibrate without deadening the sound. That could be anything from a drumhead to a sheet of steel, but it must be able to vibrate effectively, since that will be where the sound of the vibration becomes heard. Your choice here will determine many of the overtones of the instrument, so if you can, experiment and see what sounds you like.
Shape Matters
The shape of your resonance chamber will affect the overall sound too, but not as much as its size and shape. Some people swear by one shape or another, but as long as the area remains more or less the same, only extreme changes in shape will affect the sound of the instrument dramatically. One example of an extreme shape change would be to replace the resonator of the guitar with an equivalent volume of 6 inch PVC pipe with drum membranes on the top an bottom. The guitar would tend to want to vibrate at the fundamental frequency of the PVC pipe, giving it a very weird sound. Still, that is an extreme example. Most "boxish" shapes will be similar. Experiment until you find the sound that suits you, and be prepared for amateurish sounds at first.