So, there are some people that claim that packing a duffel bag is like playing a game of Tetris. I have to disagree. The reason for this is not because of the sheer amount of spatial relationships you have to consider when packing a duffel, that is very true, and somewhat similar to Tetris. However, in Tetris you don't get the chance to start over and start a new pattern to see if you can cram more crap in by shifting things around. As my friend Sean once pointed out: "Everything in the Army is just a little too small." You can almost get all of your stuff in a duffel. And if you decide to pull something out, it usually ends up looking like a half-deflated balloon, and there's something about the top latch on a duffel that makes you worry that unless it is packed tight and snug, the bag will open up at the worst possible moment, and you'll be one of those unfortunates whose clothing comes out separately from your bag at the airport.
Instead, I think that packing a duffel bag is a lot more like trying to figure out how to power up a lunar module without shorting out the electrical system and stranding three astronauts in the "depths of space" (trademarked phrase by the way). You have to figure out which combination will optimize the regular volume of the duffel compared with the irregular volume of all the items that go in the bag. Fail to come up with the right combination, and you're stuck with a leftover boot, shirts, or possibly even a helmet. I have packed and repacked dozens of duffel bags over the years, and this always holds true. The bag is just a little too small. But you unpack and repack it, hoping against hope that maybe if you shift your boots 90 degrees, put your books in vertically instead of horizontally, and maybe, just maybe, you can cram two more pairs of socks inside the helmet and open up some space for that last shirt . . .