Yesterday I picked up a stainless steel, six-ounce, hip flask at Hi-Time, along with a small funnel to aid in filling it.
Right away I discovered some useful information. A glass flask would have the advantage of allowing one to see when it's about to overflow while being filled.
Given the need to rinse the inside occasionally, a permanent, leather covering would be something of a disadvantage. (Mine's cheap-looking plastic, which will probably crack and peel someday, but I wanted something less conspicuous than stainless steel.)
A captive cap is a plus overall, but it makes shaking out the water after rinsing a noisy and/or clumsy affair.
An attached shotglass, such as the one that Chuck Cowdery once described, would make the drinking more enjoyable.