You will need a large, Quaker or similar 1 minute quick oatmeal container. Make sure it has a cardboard bottom and NOT metallic! Dimensions of this oatmeal container are approximately 5" across and 9.5" length.
You then need a spring. The type of spring is crucial to obtaining the proper sound. I recommend this one from McMaster-Carr:
. The spring specs are 0.187" OD, 0.020 wire diameter, 20" length and type 302 stainless. Cheaper springs may work, but have not been tried in this diameter.
You will need a vice, needle nose pliers, and a 15 foot tape measure. Lastly, you will need a pen with a decent point.