Radar-absorbing shielding.
We have used this to do measurements with a 60 GHz radar transceiver (Acconeer XR112). Radar sensors in the 60 or 86 GHz band are used by self-driving cars to detect other vehicles (or pedestrians) around them. A transceiver emits a radar pulse, and detects the reflection from objects around it. The time that the radar wave takes to be reflected is then used to measure the distance to the objects.
When doing tests in a lab, it is important to measure only reflections from the object, and to avoid reflections from the table, wall etc. This is where this shielding comes into place, because its sharp pyramids are designed to scatter and absorb the radiation rather than reflecting it. We printed several of these plates and mounted them all around the experiment.

Important: print this shielding from ESD-conductive filament, and mount it on an aluminum plate. Radio frequency waves will then be scattered into the valleys and absorbed. The metallic plate shields the experiment from any outside sources.