Hey. Just a thought: Flip Flops don't weigh a lot and take up very little space in a backpack. Their main drawback is that they "flop" around as you walk. All the wear and tear on your feet occurs as they rub. If you duct taped the flip flop to your foot, or alternately, made a shoe out of the flip flop and duct tape, you could have a cheap, light weight emergency back up. Slippers and sole inserts would work well, too.
I have wanted to make my own pair of shoes for a while, now. I wanted a slipper with a slightly thicker, but still soft, sole.