<<
  \new Staff = "main" \with {\remove "Time_signature_engraver"} 
  { 
    \clef bass
    g,1 
    <<
    { g,1 g, } % main staff notes, equal length to ossia section
    \new Staff = "ossia" \with
    { 
      fontSize = #-3
      \override StaffSymbol #'staff-space = #(magstep -3)
      \override StaffSymbol #'thickness = #(magstep -3)
      \remove "Time_signature_engraver"
      alignAboveContext = #"main"
    }
    {
      \clef bass
      c1 c
      \stopStaff
    }
    >>
    % back to the main staff notes
    g,
  }
>>