technique - What is the best practice for writing a character that speaks with a whistle lisp?

For a character that whistles when pronouncing the "s" sound, how can I write in the lisp effectively?

I tried using multiple "S"s, but it reads as a snake-like sound, which I don't want since there are elements of the character being snake-like as well.

How do I make the whistling "s" lisp really stand out?

Answer

How about going with this♪?

Possibly with the note raised as superscript. After an initial description it serves as a reminder to the reader.