The main inner boundary portion MI may be formed as a single wall structure between the first and second accommodation spaces G1 and G2 adjacent to each other in the second direction Z2 and between the third and fourth accommodation spaces G3 and G4 adjacent to each other in the second direction Z2. Also, the main inner boundary portion MI may be formed as a dual wall structure between the first and fourth accommodation spaces G1 and G4 adjacent to each other in the first direction Z1 and between the second and third accommodation spaces G2 and G3 adjacent to each other in the first direction Z1. According to some embodiments, the dual wall structure between the first and fourth accommodation spaces G1 and G4 may be arranged with the circuit space CS therebetween. Likewise, the dual wall structure between the second and third accommodation spaces G2 and G3 may be arranged with the circuit space CS therebetween.