When I make French bread I like to shape some of the dough into rolls (petit gras batards) to use for Hoagies. They're very good, but there is one problem with them: When you bite into the sandwich, the filling tends to try and escape out the sides. I was thinking that a cavity in the rolls would provide space for the filling, and minimize this effect.

Does anyone have a suggestion as to an object I might put in the dough, before baking, so that when the object was removed from the baked roll after slicing it that there would be a cavity running almost the full length of the roll? I thought in terms of a ceramic rod, or a cylindrical stone, but couldn't come up with anything specific.