Compound structures and methods for forming the same are described. The compound structures can be used to form an enclosure. The enclosure may be formed from metal, such as aluminum, and further include one or more non-metal regions that allow for transmission and receipt of electromagnetic waves, such as radio frequency waves. The non-metal region can include a first section, a second section, and an optional cosmetic section. The first section can be firmly molded onto a metal section of the enclosure by small pores formed within the metal section. The second section can engage with interlock features of the first section. The optional cosmetic section can cover the first section and the second section such that the first section and the second section are not visible from an exterior of the enclosure.