A bar-mounted filing vise is the answer.

A bar-mounted filing vise is the most stable way to clamp a chainsaw bar for hand filing in the field, because it turns any stump or log into a workbench.

The whole problem is that you don’t have a proper workbench out there. You could try clamping the bar with a C-clamp to a log or a tailgate, and that works in a pinch, but C-clamps can bend or scar the bar if you lean on them, and you still have to find a surface that lines up right. A filing vise is built for this job: it grips the bar firmly, sits on the stump by itself, and lets you set the bar at whatever angle feels comfortable.

If you don’t want to carry dedicated gear, the next best trick is to wedge the bar into a cut in a log. Make a couple of saw cuts in a stump, slip the bar in, and tap some wedges in beside it. That’ll hold it steady enough for a quick touch-up. But if you’re doing serious hand filing, get the vise. It’s small, cheap, and will outlast every chainsaw you own.

Your chain will thank you, and your knees will too.