The styrocello is a budget musical instrument that can be created with some simple parts -- junk, even -- but creates a satisfying tone with minimal effort. Whoever said you couldn't build your own musical instrument?
The styrocello was first conceived of by Skip LaPlainte, who builds and composes for instruments made of entirely trash. Using stuff you might find in a dark alley, you can probably get all the equipment for free, or if not, on the cheap.
You need a poster tube (preferably as thick as you can find), a styrofoam box, a wire, and the tools to put it together. The basic formula involves knifing circular holes on the sides of the styrofoam box, sticking the tube through the box, and bolting the tube to it. Then attach the wire to the bolts. Alternatively, you can use a wooden block for the bridge of the instrument.
Mostly, the sound of the instrument is determined by experimentation. For instance, the thinner the gauge of the wire you use, the quieter the instrument will be. However, the string will also snap sooner. Though it is easier to only use one string, you could theoretically design a multi-string model.
As for the bow, you could cheat and use a violin bow or a cello bow. It seems difficult to replicate the sound of horsehair or other fine material with junk, but certainly there is some magical alternative that your friends and neighbors throw away every evening.
It seems silly, like the rubber band guitars that get built by kids, but the magic is that this is a real string instrument with a quality that entirely outstrips its price. The only important elements are the styrofoam to serve as a sound board, the cardboard to serve as a neck, and the wire to serve as a string.