Is it possible to print a resonator for a musical instrument?
I can't answer this from a technical 3D printing angle.  But, from a musical angle:

Where the body of an instrument has the primary function of enclosing a vibrating air column, the material has often been demonstrated to perhaps make a difference, but only a subtle one.  As an example, a recent range of plastic trombones, although not first-class instruments, have proved extremely playable (and have the great advantage of being virtually indestructable). 

I suggest you try this.  Play your instrument, dampening vibration of the cabaça with your hand or with a piece of cloth.  Just damp the shell externally.  Don't obstruct the hole or put anything inside.  Now, fill the cabaça with cotton wool or similar.  If the first makes little difference, you're probably good to go with a plastic cabaça.  Of course, if the second makes little difference either, we might have to suspect that the cabaça is mainly decorative!

You could also experiment with alternative resonators of a similar size and capacity, available 'off the shelf', not worrying too much about a cosmetic match.  They might sound even better!