The advantage of a small mirrorless system means that you can basically just buy a small camera insert, and fit that to any small bags.
I got myself a small Puleme (Korean brand) camera insert for less than $20, and have been using it for a small mirrorless and 3 lenses. It goes very well with any small messenger bag. If you need to bring along a tripod, then you can use the same camera insert and slot it into a backpack rather than a messenger bag.