Carbon. The outer shell does not need to be a single structure. A swam of micro satellites orbiting at 25 AU with 1 atom thick carbon antenna collectors (think

Carbon. The outer shell does not need to be a single structure. A swam of micro satellites orbiting at 25 AU with 1 atom thick carbon antenna collectors (think black snowflake) would on average be not much more than 1 atom thick and would collect 99% of the energy emitted by the star or inner shell(s). This is all rather beyond what my post was about which is that only inefficient Dyson spheres can be detected in the infrared and that any civilisation capable of building a 95% efficient sphere is able to build one that is 99%.