Sorry to be "late".
There´s a clever way! Use a dental floss (sorry about my spelling) "box".
Open carefully so you can close it (or glue if its the case) and roll the flash string in the place.
I did it and it works, altough I don't use flash string as often as you do, it is replaceble (?does this word exist?). I sugget painting it with a sharpir so it becomes all black and then doesn't look like you are going to the toilet.
If the size you want if very specific and you want to carry then separeted and not cutting then one at a time you could think again. there are good ways of knowing the right size by the pocket you leave it, or you can use your arm or fingers, I don't think that would be a problem.