The present invention is directed to an improved container closure or lever cap comprising one or more of the following features: (i) an improved interface on the interior surface of the closure that consists of strategically positioned locking lugs to securely affix the closure onto a container, (ii) a repositioned lever handle and fulcrum to optimize function and mechanical advantage, (iii) a retro-fit sleeve for an incompatible container, which on the inner surface is fashioned to interface with twist necks bottles, or other closure designs and mechanisms incompatible with the interior fitting of the lever cap, and which on the exterior is fashioned to receive the interior fitting of the lever cap, (iv) a tamper-evident safety indicator, (v) a specialized vertical sloping sidewall at the container's distal end near the opening to improve the closure's vertical lift during operation, (vi) a child safety design and mechanism that requires two functions to unlock the lever and position it for normal operation, and (vii) certain ergonomic, aesthetic, and customizable features that improve its function and consumer appeal.