Unfortunately, Enfold 3D does not currently support a thickness function.
You can export the 3D model from Enfold 3D and bring it into Strata Design 3D CX where you can add elements to mimic thickness. However, it can't be done directly in Illustrator with Enfold 3D.
Enfold 3D is developed by our partner EskoArtwork. They do offer products that allow you to set the thickness of the material. These higher end products are predictably more expensive - but excellent for some of these types of functions.