The idea that vacuum can « fluctuate » and that vacuum has a background energy are simply wrong. These ideas are opposed to the definition of vacuum and are, so, irrefutable. There are many very simple ways to actually explain the Casimir effect.
The most simple is photons in the experiment, for instance the light needed to observe the evolution of the plaques : in a vacuum, the pressure of the photons is enough to bring the plaques into each other. Photons have a mass – they would not have any existence without it. The use of laser beams to measure the effect is an even worse example (where the laser directly impacts one of the two plaques, or another object tested like a sphere with a cantilever onto which the laser is beamed…).
The presence of neutrons from solar activity in the experiment area is another trigger for the movement of the plaques.
Another possibility that can be raised is the presence of alpha and beta emitters, if the vacuum is slightly imperfect. The energy of their decay will contribute to the clustering. Most of the radioactive nanoparticles will be outside the plaques and their energy will bring the plaques together.
Another possibility is the background gamma radiation and its reflection on the sides of the experimental set-up. Gamma photons bouncing on the sides can contribute to bringing the plaques together. The presence of uranium, thorium, etc. in the concrete of the walls of the building in which the experiment takes place can also contribute.
In all cases, each plaque protects the other one and it is impossible that the inner sides of the plaques receive more particles than their exterior sides, so that this initial set-up of plaques explains why the plaques move into each other and do not go away.
The ideas that vacuum has a background energy and can fluctuate are opposed to the definition of what vacuum is. If vacuum has an energy, this energy needs a support, a carrier, and this means that vacuum is not vacuum. An « intrinsic movement in vacuum » is a phrase impossible to write ; it should be written « an intrinsinc movement of a few sparse particles in a quasi-vacuum ». These particles can be, simply, radon 222 & radon 220, or solar neutrons, or photons, or other forms of contaminants.