A buckled sheet offers a reservoir of material that can be unfurled at a later time. For sufficiently thin yet stiff materials, this geometric process has a striking mechanical feature: when the slack runs out, the material locks to further extension. Here, we establish a simple route to a tunable locking material: a system with an interval where it is freely deformable under a given deformation mode, and where the endpoints of this interval can be changed continuously over a wide range. We demonstrate this type of mechanical response in a thin sheet formed into a cylindrical shell and subjected to axial twist and compression, and we rationalize our results with a simple geometric model.