The widths of the top fastening pillars LU1 to LU4 based on the XY plane may be greater than the widths (or thicknesses) of the side covers CV_SF1, CV_SF2 and CV_SF3.
The first enclosure 121 may further include one or more inter-device fastening pillars DL1 to DL3 in which fixed holes DH1 to DH3 are placed, as illustrated in FIG. 4. The inter-device fastening pillars DL1 to DL3 protrude downward from the lower face BF of the first main cover CVU, like the top fastening pillars LU1 to LU4. The widths of the inter-device fastening pillars DL1 to DL3 based on the XY plane may be greater than the widths (or thicknesses) of the side covers CV_SF1, CV_SF2 and CV_SF3. The heights of the inter-device fastening pillars DL1 to DL3 are greater than the heights of the top fastening pillars LU1 to LU4 and may be greater than or equal to the heights of the side covers CV_SF1, CV_SF2 and CV_SF3. The planar shapes of the inter-device fastening pillars DL1 to DL3 may be, but are not limited to, a polygon such as a square, a rectangle or a rounded rectangle.
