Review: Ortlieb Ultimate Six Classic 5L
This is an excellent waterproof handlebar bag from Ortlieb.
Even the smaller 5L version is pretty roomy. (Credit: Ortlieb. Fair use.)
It has room for any utensils and snacks that you might want access during your ride (and there exists an even bigger 6,5L version too). The topside has large window so that you can put your phone in there in landscape mode, and use it as a navigator for example. Build quality is excellent, and it even comes with a shoulder strap for use off the bike. Just remember that you have to get the mount separately.
Highly recommend this one!