I used to carry a cassette cracknut, like the one pictured.

Then I got a bender that I could use to dog-leg my spare spokes. I got this at a hobby shop. It's designed to put a dog-leg into RC airplanes' throttle wires. It works perfectly on bicycle spokes. (I added the pink line to demonstrate the shape on the working end of the bender.) With dog-legged spokes, no cassette removal is needed to replace them. Also attached is an image of a dog-legged spoke that I replaced.

I also carry a fiberfix spoke so I can put off actually installing the replacement spokes until I reach my destination.