[sdiy] vocal-tract filters

[MED]

A while ago, the chatterbox article was posted and i read it with great 
interest. In the article, it said that the filters it was using to simulate 
the vocal tract were just approximations, because they didn't want to go 
through the effort to get it right :)
So does anyone know where to find a vocal filter that *does* get it right (or 
as close to "right" as possible)?
VC would be nice, though not *expressly* required. Tuning stability, other 
than "no drastic drift that can be heard during the duration of a sound," is 
not an issue.
Thanks in advance,
MED