Can you put the plate in a frame and then cut the bottom of the plate to accommodate that sensor without it being completely obvious? A plastic frame is probably radar transparent, so if you cut the plate and left the frame intact it would hard to detect and also might not compromise your detection range that much. And if you need LIDAR pass through, you could probably put some dark LiDAR transparent plastic over the cutout in the frame.
Like this stuff:
https://solutions.covestro.com/en/hi...makrofol-st352
Edit: looks like this shop did a custom license plate frame to accommodate a sensor:
https://www.rdforum.org/threads/110957/