When I try to add a radiation thermal load to my model, I get the sidebar, I can choose surface-to-surface, but I can't pick any geometry to identify these surfaces.  I'm modeling a vacuum flask (like a Thermos), so there is vacuum between the inner and outer "bottles," and radiation becomes significant.

Also, I'd appreciate other opinions, is there any better way to model the vacuum than to create a 3D solid in the cavity and assign it to be modified air with minimum (1e-15) mass density and thermal conductivity?