Assemblies that can be disposed in a subterranean bore are described. An assembly can include a recessed portion in an inner wall and another recessed portion in an outer wall. The recessed portions can each be configured to have a cross-sectional thickness that is less than at least another part of the assembly. The assembly can provide a seal between an inner region defined by the assembly and an environment exterior to the assembly prior to a window being created in the recessed portion in the outer wall through which a branch wellbore can be formed. The recessed portion in the inner wall can guide a cutting tool toward the recessed portion in the outer wall.