Stem bag

I consider these an invaluable bikepacking bag as they offer great on-the-bike access and quite a lot of volume. On my own bike I have one for a 750ml bottle on one side and a lidded version with food and essentials like sun cream on the other side.

These come with Voile straps for mounting them to the bar and two extra straps (one around the stem and one around the fork leg) for ultimate stability.