K, a good question which I have not seen here before. Here's how I keep my frequently used brushes--a canvas carrier that easily fits into my backpack for plein air, workshop and travel uses. As one can see, this carrier is well used and contains all of the brushes I may need, regardless of intent or size of painting.
I actually have two of these, each containing a variety of brushes. To the far left one may glimps seldom used brushes in large containers.
Hope this helps!