I think you just want to cool the camera in the box by circulating cooled around it or by conducting energy by brute force, per my diagram - though uneven cooling might be a problem. Adding more metal inside will diminish the TEC performance overall. TEC performance is basically a matter of moving as much heat as possible from the hot side irrespective of how cooling is performed on the inside. Ambient /Fan exhaust deltaT <10C is good.
EDIT: If the hotside heat sink/fan combo is too small/inefficient, best TEC performance occurs at a lower voltage/current than for a more efficient combination. In this case adding more energy further reduces the cooling performance of the TEC. This needs to be resolved before considering how to obtain thermal performance on the inside. Heatsink, fan and good clean flat junctions will lower thermal resistance, by comparison, and improve the chance of better internal cooling
