A well system includes a casing string extendable into a primary wellbore from a surface rig, and a sidetrack casing assembly coupled to the casing string. The sidetrack casing assembly includes a cylindrical housing that defines a window through a sidewall of the housing, and a cover removably coupled to the housing at the window. The cover is transitionable between a closed state, in which the cover is attached to the housing and occludes the window, and an open state, in which the cover is moved to expose the window for drilling a secondary wellbore from the primary wellbore.