The Slope are should be smooth and the point where the angles change and the slope begins should be a smooth transition. If not, the sound will be unstable.
The FIMO Modeling Clay can be obtained at Walmart stores in the crafts department.
If you plan on building more instruments. A Dremel is very useful for shaping the plug made of FIMO or Sculpey (Polymer Clays). .
Note: Wood can be substituted for FIMO plug. Sculpey Modeling compound can be used as well. If you break through the thin overhang area, take a wood dowel that fits snuggly into the pipe and drill a wide hole through it so that air can freely pass through and place into the air where the over- hang should go. (Use CPVC orange colored cement)
TONE BODY DIMENSIONS TO MAKE A COMPLETE INSTRUMENT
Enjoy, Let Me know if you build one.