I just recently bought an REI Zip Travel Daypack. The official measurements: 18"x10"x6.5". As I loaded it, it's a little closer to 20"x10"x7". One of the benefits of this size bag is that, even loaded to the gills, it meets most American airlines' requirements to qualify as a "personal item" (there's no official definition, but in what I've read, anything less than 38 linear inches generally qualifies), meaning that you can slide this bag under the bottom of the seat in front of you while bringing aboard another slightly larger carryon that measures up to 45 linear inches (generally 22"x14"x9"). Plus, it retails for $29.50. I had carried this little bag, loaded, in the trunk of my hybrid for several days but tonight noticed that it appeared made for the shelf created by the pass through from the trunk to the rear seats.

And, so it was! The bag wedges in almost perfectly, with it snug along its width and depth. It appears there's an inch or two I could play with along the height to go to about 22" but otherwise, this bag fits into that space so snugly that it's not going to pop out no matter what bumps I hit. I'm happy about that because I've been trying to figure out the best use for this space. And since I can pack this bag tightly with enough to last me for a three or four-day golf trip, I can leave more space in the trunk for the essentials--like golf clubs.

Of course, any bag with roughly the same measurements would work the same way. And, I've looked through the Toyota manual and haven't seen any prohibition against leaving an item in this space for an extended period of time, unlike the warning not to cover the exhaust vents on the shelf behind the passenger seats. I can't wait for my next road trip!