The satellite structure has a number of functions, the most obvious being to hold all the components together through the stresses of launch. But there are other functions as well, such as to support for the thermal control materials on the surface of the satellite and provide a level protection from ionising and electromagnetic radiation for the electronics.
The structure consists of two parts - the mechanical supporting structure which can be see in the upper image and an outer skin which is thermally isolated by teflon spacers, clearly visible in the lower image. Interestingly, the magnet of the Stabilisation and Orientation system is visible sticking out from below the batteries (along the x-axis). The hysteresis rods were glued to the inside of the upper skin of AO5.
The material chosen for the structure, including the satellite skin, was a commonly available aluminium alloy containing magnesium and silicon and small amounts of copper and chromium. The alloying elements improve corrosion resistance and strength.