The trend for bikepacking at the moment seems to be i favour of something like this which is available in several sizes.
Topeak Backloader
There is also this style of bag which fits to your seat post with a quick release adapter which is again available in 2 sizes. It's referred to as an off-road bag but don't let that put you off - the bag is just as happy on road as off.
Vaude Website
Vaude Seatpost bag
There are a huge number of bags around in different styles etc so you should be able to find one that suits. The reason why I've linked to these two in particular is that I have both of them but other manufacturers do similar.
I find with the Vaude style of bag that it is much quicker and easier to unzip the bag and get out what I want. It can be removed from the bike and refitted easier as well. With the Backloader style bag there is just a bit more of a faff involved with doing this.
Importantly the Vaude is a rigid style of bag so so less likely to squash those sandwiches
One thing you do need be aware of is that Vaude state that it shouldn't be used with a carbon seatpost because the 'quick fit' clamp tightens on to the post. I guess this is where the straps of the Backloader style of bag do have an advantage.