The Karman Ghia also has much smaller tail lights. It's not the material so much as if it is long enough. You don't want to have to glue several pieces together. Ideally, the seal wouldn't have any cuts.
The American Rabbit taillight seal might be long enough because it's also a huge tail light.
As Scott mentioned, you can find similar material at Home Depot or Lowe's. I have a shower that needs sealing and they have a product called "Backer Rod" that you use to provide a sound surface for caulking.
EDIT: They also have this:
You could also buy a used tail light from eBay just for the seal.