I have one from cannondale that looks like a thermal watter bottle sleeve that attaches like any other bag behind the saddle.

I use in on really hot days where I think I might need three bottles of H20, but that does not happen too often, so I just use it to carry stuff.

It is about 7 years old and have not seen too many like it.

Hope this helps,

ehenz

ps. there is also those camel back things?