You can use a variety of breads for this tasty snack, but I would recommend one made with sourdough for the balanced flavor. My aunt uses toast rusks/Zwieback instead of bread, which is also surprisingly delicious.
You can also omit the baguette altogether. To do this, simply use two slices of eggplant (and slightly bigger ones) instead of one. So it is then stacked: eggplant slice, garlic mayonnaise, tomato slice, garlic mayonnaise, eggplant slice. And now you have great antipasti!