So, this is what I needed for three days including my purse (the red bag) which has my makeup inside:
One handy tool for efficient packing is the shirt folder by Eagle Creek. You fold over the board, then velcro the wrap around the stack. I also use their packing cubes for longer trips. You can get these online or at the Container Store.
Then it is just a matter of finding the right size bag. This one by BlueQ is a little smaller than necessary. However, the Spirit size limit is smaller than most airlines and it is hard to find bags under 18".
Once upon a time, I packed for every contingency, but luggage was always a hassle. Now, I'd rather have less choices or have to wash something than lug around large pieces through an airport!