The present disclosure includes devices, apparatuses, kits, and methods for replacing a nucleus pulposus of an intervertebral disc with an implantable nuclear prosthesis filled with a curable silicone material in situ. Configurations of the present spinal implant devices include a flexible body defining an outer fillable enclosure that defines an outer chamber, an inner fillable enclosure that defines an inner chamber, and a proximal plug configured to be coupled to the inner fillable enclosure.