We study Winter or [Formula: see text]-shell model at finite volume (length), describing a small resonating cavity weakly-coupled to a large one. For generic values of the coupling, a resonance of the usual model corresponds, in the finite-volume case, to a compression of the spectral lines; for specific values of the coupling, a resonance corresponds instead to a degenerate or a quasi-degenerate doublet. A secular term of the form [Formula: see text] occurs in the perturbative expansion of the momenta (or of the energies) of the particle at third order in [Formula: see text], where [Formula: see text] is the coupling among the cavities and [Formula: see text] is the ratio of the length of the large cavity over the length of the small one. These secular terms, which tend to spoil the convergence of the perturbative series in the large volume case, [Formula: see text], are resummed to all orders in [Formula: see text] by means of standard multiscale methods. The resulting improved perturbative expansions provide a rather complete analytic description of resonance dynamics at finite volume.