I had to make part of one for an old Grand Ville, so I went by the local Chevy body shop & did a little dumpster diving. I found a very slightly damaged but big (and free) taillight from a wrecked Chevy, and glued pieces of it where the Pontiac lens was broken. You just need to find a light that's bigger than the Rambler lights.
Or, you might drop by a parts store (or Wal-Mart) and see what they have in truck/trailer tail light lenses.
My utility trailer has oval tail-light lenses that might be around the right size. What's the height & width that you need?