A backrest member (e.g., a portable insert) for use with a seat member of a chair. The backrest member may include an adjustable platform attached to a substrate (e.g., frame) of the backrest member, and an adjustment mechanism configured to adjust the adjustable platform along one or more locations of the platform to provide a desired contour for supporting the back of a user seated in the chair. The adjustable platform may comprise an elongate band attached to the substrate of the backrest member that is disposed along a central longitudinal axis of the backrest member. One end of the elongate band may be attached to the substrate, while an opposite end may be slidably attached to the substrate. In an embodiment, both ends may be slidably attached relative to the substrate. The device allows selection of a height and depth of a lumbar curve provided by the device.