How to add padding around whiteout text

[Joseph N. Srednicki]

Hello:

Please see the following code example. 

In this example, I used the \whiteout command so that my markup looks like
it interrupts or a layer on top of the spanner bar.  

Could someone please explain how to add some padding around the text or
point me to the location in the documentation or mail archive that deals
with this issue?

Thanks for your help.

Joe Srednicki 

==================
\version "2.19.35"
\language "english"
global = {
  \key c \major
  \time 4/4
}
rightOne = \relative c'' {
  \global
  c c c c
  c c c c
}
rightTwo = \relative c' {
  \global
   e e e_\markup {\whiteout {\fontsize#-3 {ad libitum}}}  e 
   e e e e
}
leftOne = \relative c' {
  \global
  % Music follows here.
  g g g g
  e e e e
}
leftTwo = \relative c {
  \global
  c c c c
  e e e e
}
\score {
  \new PianoStaff \with {
    instrumentName = "Pno."
    shortInstrumentName = "Pno."
  } <<
    \new Staff = "right" << \rightOne \\ \rightTwo >>
    \new Staff = "left" { \clef bass << \leftOne \\ \leftTwo >> }
  >>
}