  It is shown that exact spherically symmetric solutions to Einstein's Field
Equations exist such that, over an open region of the spacetime, they are
singularity free, satisfy the dominant energy condition, represent elastic
matter with a well defined constitutive function, and are such that elastic
perturbations propagate causally.
  Two toy-models are then built up in which a thick elastic, spherically
symmetric shell with the above properties, separates two Robertson-Walker
regions corresponding to different values of the curvature $k$. The junction
conditions (continuity of the first and second fundamental forms) are shown to
be exactly satisfied across the corresponding matching spherical surfaces.
