  In this paper we study the Casimir energy of QCD within the Gribov-Zwanziger
approach. In this model non-perturbative effects of gauge copies are properly
taken into account. We show that the computation of the Casimir energy for the
MIT bag model within the (refined) Gribov-Zwanziger approach not only gives the
correct sign but it also gives an estimate for the radius of the bag.
