When I was in the military, it was always necessary to pack light, or improvise.
Portable vices are something that you should not be without. I learned that years ago. These things are not heavy duty, but are great when you need a third hand, balance or equipment holder. I have two, and use both regularly.
Paired with slip on soft (rubber) jaws, they can hold anything without damaging metal or gouging finishes. It is quick to clamp on to my benches. It swivels, tilts and is inexpensive. The second picture shows the soft jaw inserts.
Over the years, I have used this to hold numerous rifle barrels, stocks, triggers, gauges, tools, flies, and my cell phone. I never realized how handy it was until I didn't have it on a hunting trip. One always goes with me.
These are available from amazon, Princess Auto, Crappy Tire and hardware stores. Here's a link that you can use to compare the different ones.
Portable Vises