Creating a custom clef glyph

This snippet shows how to print a new clef glyph - here an old style bass clef - with a path encoding (support svg output).

\version "2.24"

%% http://lsr.di.unimi.it/LSR/Item?u=1&id=900
%% <= http://lilypond.1069038.n5.nabble.com/Old-Bass-Clef-td28833.html
% add by P.P.Schneider on March 2014. Edited on June. 2017.

%% Path drawing:
my-clef =
#(ly:make-stencil
  `(path 0.001
     (moveto   -0.10  -0.35
      curveto  -0.10  -1.11   0.48  -1.80   1.25  -1.80
      curveto   1.75  -1.80   2.20  -1.60   2.50  -1.05
      curveto   2.55  -0.95   2.50  -0.90   2.43  -0.95
      curveto   2.20  -1.17   1.90  -1.48   1.45  -1.48
      curveto   0.70  -1.48   0.15  -0.85   0.15  -0.15
      curveto   0.15   0.45   0.62   0.83   1.05   0.83
      curveto   1.55   0.83   1.90   0.50   1.90   0.05
      curveto   1.90  -0.35   1.55  -0.71   1.20  -0.71
      curveto   0.80  -0.71   0.60  -0.40   0.63  -0.17
      curveto   0.75  -0.30   0.90  -0.31   1.00  -0.31
      curveto   1.20  -0.31   1.38  -0.10   1.38   0.15
      curveto   1.38   0.35   1.20   0.55   0.95   0.55
      curveto   0.60   0.52   0.39   0.25   0.39  -0.05
      curveto   0.39  -0.52   0.62  -0.89   1.20  -0.89
      curveto   1.65  -0.89   2.07  -0.50   2.07   0.10
      curveto   2.07   0.66   1.56   1.01   1.05   1.01
      curveto   0.28   1.01  -0.10   0.35  -0.10  -0.35
      moveto    2.42   0.20
      curveto   2.54   0.20   2.64   0.30   2.64   0.42
      curveto   2.64   0.54   2.54   0.64   2.42   0.64
      curveto   2.30   0.64   2.20   0.54   2.20   0.42
      curveto   2.20   0.30   2.30   0.20   2.42   0.20
      moveto    2.42  -0.20
      curveto   2.54  -0.20   2.64  -0.30   2.64  -0.42
      curveto   2.64  -0.54   2.54  -0.64   2.42  -0.64
      curveto   2.30  -0.64   2.20  -0.54   2.20  -0.42
      curveto   2.20  -0.30   2.30  -0.20   2.42  -0.20
      closepath)
     round round #t)
   (cons -0.1 2.65)
   (cons -1.3 1))

\layout {
  \context {
    \Score
    \override Clef.stencil = 
      #(lambda (grob) 
         (let* ((sz (ly:grob-property grob 'font-size 0)) 
                (mlt (magstep sz))
                (glyph (ly:grob-property grob 'glyph-name))) 
                 (cond
                  ((equal? glyph "clefs.F")  
                     (ly:stencil-scale my-clef (* 1 mlt) (* 1 mlt)))
                  ((equal? glyph "clefs.F_change")  
                    (ly:stencil-scale my-clef (* .8 mlt) (* .8 mlt)))
                  (else (ly:clef::print grob)))))
  }
}

%% Test:
#(set-global-staff-size 30)

\new GrandStaff <<
  \new Staff { 
    \clef G c'1 
    \clef F c 
    \clef G c' 
    <<
      a,
      \new CueVoice {
        %% Does not work:
        %\cueClef F
        \once\override Staff.Clef.font-size = #-4
        \clef F 
        f^"cue"
      }
    >>
  }
  \new Staff  {
    \clef F c1 
    \clef G c' 
    \clef "F^8" c' f
  }
>>