Man, I really anal about shit like this, so take this with a healthy shake of salt.
If you don't have a really nice file, I would spend the $10 or so and buy one. I would most likely buy a new one anyway if I didn't have one that was the perfect size and shape for the job. I would want a flat second cut or smooth cut, maybe even both.
I would then measure the OD of the headtube, and make some sort of gage out of plastic or metal. I've found that to me at least, my eyes try to fuck me when it comes to making things round.
Finally, just go to work on that shit. I would try to draw file, and check with my gage to make sure I'm removing material from the right place every 10 strokes or so.
Once it's pretty close, I would hack together a sanding block that matches the head tube OD, and finish it with that. If it's an externally machined head tube, you could most likely find a sandpaper grit that would match the tooling marks, and make it just about undetectable.
Like I said though, I tend to nuclear engineer the fuck out of shit like this.