Hi Richard.
Good advice above.
I now make my own Whistle bodies to go with Generation Heads.
I use the Polystyrene tube readily available in UK Model shops.
Evergreen 1/2 inch Tube No 236
This fits a Generation D or Eb top.
I'm the guy that uses bluetac to fill in the space behind the fipple to sharpen the top octave relative to the bottom.
TTFN
PP