An implant for performing interbody fusion within a human spine, inserters for such an implant, and associated methodology. The implant is preferably formed in situ from at least two separate but lockable members (a base member and a closure member). The base member may be implanted into an interbody space first, after which the end plates may be finally prepared and the base member packed with fusion promoting substances before engaging and locking the closure member. The closure member provides structural support for the adjacent vertebral bodies (along with the base member) and may be selected after implantation of the base member having a specific length, width, height, taper, etc. . . . to ensure an optimal sizing of the implant for desired restoration of disc height, coronal taper, sagittal taper, etc. . . . .