A while back in one of the threads someone noted that an Aeronaut End Packing Cube fit nicely in the Synapse 19. I didn't rush and order one because I thought I was doing fine with a combination of WF med cube and a stuff sack. I purchased one of the cubes while at the Seattle Store (very dangerous place for an addict to visit...) and just used it to pack for a weekend jaunt to a colder climate. OMG! What a difference...I was able to easily pack 2 shirts, sleep wear, 2 pr socks, and undies in the very bottom of the main pocket of my Synapse without it invading the territory of the lower external pocket. The amazing part is that these are heavier weight clothes that I would not usually choose for travel because they don't compress well.
A thousand thanks to whomever posted this idea previously---if you have a S19, I strongly encourage you to give it a try!