Gentlemen,
You can't use the Malibu wagon rear fillers on the Buick. It would have to be for an Olds or Buick wagon only.
Any particular reason for that? I did notice on that site that there are three different pieces. The end caps, and the middle filler panel. The current filler panel appears to be a one-piece. Appears that it's some type of urethane, but not exactly sure. To be on the safe side, I kept the broken piece if there's any way it can be patched back in.