Here's some pics of a vise I made out of some stuff I had laying around works great sharpening in the field.
Like this, Think Ill have to make one of these up, but most of my scrounging is limited by my ability to haul(only about 1/3-1/2 a cord at a time) so I rarely need to sharpen in the field. Usually I dont even run through a tank of gas or oil by the time a cut a trailer load full.
I always sharpen chain after use at the end of the day and keep an extra for each saw with me.I have rarely used it but a couple of times after running it into the ground instead of using another chain.It does come in handy I won't run a dull chain.