insert whitespace to fill line

[Dave]

I'm trying to make music-snippet images using lilypond, but I'd like each
snippet to always have trailing "whitespace" so that each of these images is
of the same width.  Ultimately, these images will be embedded in an
emacs-org file (I am aware of lilypond-book).

I'm following the suggestions from the documentation, so I have a .ly file
something like:

    \paper{
    indent=0\mm
    line-width=120\mm
    oddFooterMarkup=##f
    oddHeaderMarkup=##f
    bookTitleMarkup = ##f
    scoreTitleMarkup = ##f
    }
    \relative c' { c4 d e f | g a b c } % I'd like to insert whitespace to
fill line here

and compile it using 'lilypond -dbackend=eps -dno-gs-load-fonts
-dinclude-eps-fonts --png test.ly' and it produces a file that is 241 pixels
wide.

If I make the snippet three bars, e.g. with c4 d e f | g a b c | c b a g, I
get a file that is 335 pixels wide.

Ultimately, I'm going to insert these images into a document, and I'd like
to be able to use the same width for all of the images, and have the
staff/notes be the same size.

I don't want to key off of the height of the images (which is the same in
this case), because some of the snippets will span multiple lines.

I don't want to pad with s1's since the empty bars will be distracting.

Is there a way to "pad" the line with whitespace to that the produced files
have the same width?