Or, if you want to keep your syntax:
\version "2.18.2"
myCluster = {
\once\override NoteHead.X-offset = #0
\once\override NoteHead.stem-attachment = #'(0.8 . 0)
\once\override NoteHead.stencil =
#(lambda (grob)
(grob-interpret-markup grob
#{
\markup {
\with-dimensions #'(-0.4 . 0.4) #'(0 . 0)
\override #'(thickness . 5)
\draw-line #'(0 . 0.5)
}
#}))
}
{
\new Voice { \myCluster <c' d' e'>4 }
\new Voice { \myCluster <a b c'>4 }
\new Voice { \myCluster <f g a>8 }
\new Voice { \myCluster <d~ e f>4 \myCluster q }
}