I was experimenting with these homemade harps because I needed a craft for a Sunday School lesson about David the Shepherd.
It's interesting to try out different thicknesses of rubberbands and see what different sounds can be made. Also, you can try different containers. I found that a really nice sound came from using a shallow microwave potato dish (see photo below) with SMALL rubberbands (so they will be fairly tight).
The big styrofoam tray (far back in the photo at the top) makes a deeper sound-- this could be a science experiment, too. If you use a larger countainer, you need longer rubberbands, and vice-versa-- if they're too loose you will not get a musical tone.