My solution for mounting a "CB" antenna was to remove the luggage rack and,
using a hole saw, cutting a hole in the bottom of the rack toward the rear,
then remounting the rack.  I use a magnetic mount through this hole to stick
to the van's roof.  If I need temporary clearance, I can just lay the
antenna on it's side.

Such a hole could also facilitate a "Permanent" through the roof mount and
the load could be unscrewed when temporary clearance is required.

Mike