Perhaps I may be of assistance.
I have a number of flasks (stainless, leather covered, etc), and the one I find most useful, *by far*, is
this one by Nalgene.
Good volume, holds almost a half of a fifth (no, that's not a tenth), and I really like the sleeve the actual flask fits into. It has a cap cover that is designed to be a "shot "glass""--silly. Unbreakable, at least I've never been able to break any nalgene bottle I have used and abused. I'm still on my first one, and I've had it for years.