\version "2.19.56"

             
#(define (my-repeat-slash-stencil grob)
  (let* (
         ;; The final stencil is a parallelogram with the coordinates
         ;;
         ;;               B      C
         ;;               --------
         ;;              /       /
         ;;             /       /
         ;;            /       /
         ;;           /       /
         ;;          --------
         ;;          A       D   

         (slope-prop (ly:grob-property grob 'slope 1))
         (slope (/ 1 slope-prop))
         (thick (ly:grob-property grob 'thickness 0.48))
         (dot-negative-kern (ly:grob-property grob 'dot-negative-kern 0.75))
         (slash-negative-kern (ly:grob-property grob 'slash-negative-kern 0.85))
         (layout (ly:grob-layout grob))
         (staff-space (ly:output-def-lookup layout 'staff-space))
         (line-thick (layout-line-thickness grob))
         ;; hard-coded as 2 staff-spaces
         (height (* 2 staff-space))
         ;;;; The angles of the parallelogram
         ;; The angle of B-A-D
         (small-angle (ly:angle (cons (* height slope) height)))
         ;; The angle of A-D-C
         (large-angle (- 180 small-angle))
         ;;;; The rectangular heights
         ;(height-A_D-to-B_C height) ;; ofcourse :)
         (height-A_B-to-D_C
           (*
             (sin (degrees->radians large-angle))
             thick))
         ;; 'height-A_B-to-D_C' is the value of interest here, it needs to be
         ;; changed to match 'thick'
         (new-thick
           (* thick (/ thick height-A_B-to-D_C)))
         (half
           (/ new-thick 2))
         ;;;; The coordinates of the parallelogram
         ;(A (cons (- half) 0))
         ;(B (cons (+ (* height slope) (- half)) height))
         ;(C (cons (+ (* height slope) half) height))
         ;(D (cons half 0))
         (x (* slope height))
         (x-ext (ordered-cons 0 x))
         (dot (ly:font-get-glyph (ly:grob-default-font grob) "dots.dot"))
         (translated-dot-1
           (ly:stencil-translate-axis dot (* 1.5 staff-space) Y))
         (translated-dot-2
           (ly:stencil-translate-axis dot (* 0.5 staff-space) Y))
         (slash
           (ly:make-stencil
            `(polygon ',(list
                         (- half) 0 
                         (+ (* height slope) (- half)) height 
                         (+ (* height slope) half) height 
                         half 0)
                      0 ;,blot
                      #t)
            (cons (+ (- half) (car x-ext))
                  (+ half (cdr x-ext)))
            (cons 0 height)))
         (slash-count
           (ly:prob-property (ly:grob-property grob 'cause) 'slash-count 1))
         ;; To get always one slash uncomment
         ;(slash-count 1)
         (slashes 
           (make-list slash-count slash))
         (slash-paddings
           (make-list (1- slash-count) (- slash-negative-kern)))
         (new-stencil
           (stack-stencils-padding-list 
             ;; axis 
             X 
             ;; dir
             LEFT ;RIGHT
             ;; paddings 
             `(,(- dot-negative-kern) 
               ,@slash-paddings 
               ,(- dot-negative-kern)) 
             ;; stencils
             `(,translated-dot-1 
               ,@slashes
               ,translated-dot-2))))

    (ly:stencil-aligned-to new-stencil Y CENTER)
    ;; sometimes better spacing with (but sometimes not):
    ;(centered-stencil new-stencil)
    ))
           
    
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%% EXAMPLES
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
    
%% Easier testing with an extern variable to set 'slope in 'RepeatSlash' and 
%% 'PercentRepeat'
mySlope = 1

\layout {
  \context {
    \Voice
    %% To distuingish between 'RepeatSlash' and 'PercentRepeat'
    \override RepeatSlash.color = #red
    \override PercentRepeat.color = #cyan
    
    %% The new RepeatSlash.stencil can be customized by overrides for
    %% slope, dot-negative-kern, slash-negative-kern
    %%
    %% Not sure why the dots do not respond for 
    %%   \set Staff.fontSize = ...
    %% Instead use
    %%   \override RepeatSlash.font-size = ...
    
    \override RepeatSlash.slope = \mySlope
    \override RepeatSlash.dot-negative-kern = 0.75 
    \override RepeatSlash.slash-negative-kern = 1.5
    \override RepeatSlash.stencil = #my-repeat-slash-stencil
  }
}

\score { 
  \relative c' {
    \repeat percent 2 { c1 } %% NB grob is 'PercentRepeat'
    \repeat percent 2 { cis2 }
    \repeat percent 4 { d4 }
    \repeat percent 4 { e8 e }
    \repeat percent 4 { f16 f f f }
    \repeat percent 4 { \repeat unfold 8 g32 }
    \repeat percent 4 { \repeat unfold 16 g64 }
    \repeat percent 2 { c4 d }
    
    %\break
    \repeat percent 4 { c128 d e f } r8 r4 r2
    
    \override PercentRepeat.slope = \mySlope
    \repeat percent 2 { c4 d e f }
  }
  \layout { ragged-right = ##t }
}

%% Also possible, from NR:
makePercent =
#(define-music-function (note) (ly:music?)
   "Make a percent repeat the same length as NOTE."
   (make-music 'PercentEvent
               'length (ly:music-length note)))
               
\score { 
  \relative c' { c4 d \makePercent s2 }
  \layout { ragged-right = ##t }
}