Travel Tool Kit: Steel Rules
Years ago, I discovered that you can buy good quality rigid steel rules through machine shop supply catalogs, and that these are absolutely straight enough to use as straightedges for fret work, layout, and other lutherie jobs.
I travel with 18, 12, and 6 inch rules in my wooden tool box, along with a 24" rule outside the box in the suitcase. Generally, it's the 24" rule that gets the most use, checking intonation, and aligning necks during neck reset demonstrations.