Trying a couple different materials for my "emergency kit highly directional #hamradio antenna":

1. Stainless steel spring-crafting wire (idea credit: @flammifer@superstimul.us) isn't the best choice of antenna material or diameter, but it is extremely portable: I'd add connectors so I could unplug the ends and twist the wire up, to have it fit in about one square foot.
2. PEX tubing is much lighter than it looks, is much sturdier and harder to accidentally deform, and covering it in copper foil tape should produce an excellent antenna. But it would be much harder to fold a PEX antenna down into an emergency kit sized package.

I think I'm just going to make both and compare them.