Solution
Standard Filler Panels are required to guarantee the cooling performance of PXI(e) systems.
EMC Filler Panels work as regular filler panels, but also contain gaskets to control EMC emissions from the chassis.

Image 1. EMC Filler Panels for PXI(e) Chassis.
If you are concerned with temperature regulation, consider using PXI Slot Blockers in empty slots, in addition to filler panels, to enhance the uneven airflow distribution caused by unpopulated slots.

Image 2. PXI Slot Blockers for unpopulated slots.