Depending on where in the world you are:

There is a resale store called Buffalo Exchange with a bunch of locations who always has the best messenger bags IMO. I’ve purchased several vintage messengers from them for carrying art supplies.

My favorite is a rigid leather satchel the size of a briefcase. Its new enough to have a laptop pocket, but old enough where the laptop would be expected to be gigantic. I use that slot for canvasses. Then it leaves the main pocket open for my mini-easel and brushes. External pockets act to replace my purse on days I carry it. All genes covered with a giant leather flap which locks closed. V protec.