So, if it is due to the virtual photon pressure
The Casimir effect is not due to virtual photon pressure. There is no such thing as virtual photon pressure and indeed there is no such thing as a virtual particle.
Electrons and photons are described by quantum fields. There is an electron quantum field and a photon quantum field and the two fields interact with each other in a way described by the theory of quantum electrodynamics. The fields have a lowest energy state that is called the vacuum state but surprisingly when the fields are in the vacuum state that does not mean the numbers of electrons and photons present are zero. Instead what actually happens is that the numbers of electrons and photons present are not defined.
This vagueness about how many electrons and photons are present is commonly described using vacuum fluctuations, but this is a rather misleading way of looking at it. If you're interested I attempt to describe exactly what is going on in my answer to Are vacuum fluctuations really happening all the time?.
Anyhow, we can assign an energy to the vacuum state called the zero point energy, and in a vacuum far from any matter this energy has a certain value. When we introduce other matter, like a conducting or insulating plate, then this changes the vacuum state because now we have to worry about the interaction with that other matter. And the change in the vacuum state changes the zero point energy.
So what is really going on in the Casimir experiment is that the vacuum state in between the two plates is different to the vacuum state outside the plates and has a different energy. Calculating what happens turns out to be a real struggle so it was done initially for the simplest system possible and this was an ideal conductor, and it turns out we get an attraction between the plates. Real metals are actually pretty close to ideal conductors so for real metals we also predict an attraction and indeed that's what we measure.
Doing the calculation for anything other than an ideal conductor is even harder, but it can and has been done and the result is that you can have no force or even a repulsive force. Indeed this has been measured experimentally and the results were published in Nature in 2009.