The instruments are feather weight. I don't know how child-proof they might be, but I am tempted to find out. I think these would be great learning tools for introducing children to string instruments.
All four sides of the harps can be played. Each side has a slightly varied selection of notes. The strings are not tunable, but there is a general progression from low to high notes.
Be sure to listen to the .mp3 audio file in the last step to hear how they sound.
Step 1: Shaping the Foam
The harps are basically Styrofoam triangles cut out of a thick sheet of the material. That gives a variety of string lengths, and a variety of notes. The longer strings make lower notes.
The strings make contact with the body only on the edges, which are protected by half-pipes of 1/2 " CPVC pipe (smallest size for hot water use). On the face of each side, between the pipes, I hollowed out the foam some, to give the strings more clearance for movement. If the strings touch anything while they vibrate, it results in a buzzing sound.
Step 2: The Nail String Guides
If you plan to alternate rows of fish line and rubber bands, make sure you don't get carried away and wrap fish line where the rubber bands go, too. If strings touch while vibrating, the resulting sound is not as clean.
I wrapped the fish line by hand, stretching it as tight as I could. It may stretch and get lower in tone over time, eventually needing re-stringing. Fish line is cheap.
Step 3: Stretching the Fish Line
When you wrap the fish line, wrap it as tightly as you can by hand. Do not release tension with your hands until you reach the end and have it tied off securely. If it slips before tie-off, you may have to go back and wind it all over again from the beginning to regain the tension.
I used two slits cut in the ends of the corner protectors to begin and end the line winding. The slits help hold the line tension while I tied the knots.
Step 4: Hear the Harps
I hope you enjoy this cheap, but pleasant sounding instrument. It's fun to play.