I need to make a homemade instrument for physics, help?

I can think of several cheap and quick options.

A guitar or harp made out of a cardboard box and rubber bands.

A Popsicle stick kalimba.

A kazoo from wax paper and a comb.

A xylophone type thing made from different sizes and lengths of plastic pipe.

Plastic party cups produce of a note of sorts if you tap them on the side with a spoon. If you add some water, the pitch of the note goes up. Get several of these cups, fill them all with a little bit of water and then "tune" them by adding or taking away water. Once you have all the notes right, just tap out the song.

For anything else, the Google is your friend.

Make a pan flute out of various length PVC tubing. Easy enough to find what length/diameter you need for each note, and then cut the tube to length.

how about a flute... simple... just a tube with a whistle at one end and a few holes in the stem... visit a musical instrument shop and do some research !