It's not the stem clamp that's the safety bottleneck, it's the bar.
And I really do suggest you use something else than a rat tail file. Even a broom handle with several layers of abrasive paper around it would be a better choice.
Put the stem on loosely and spin it. When resistance drops too much, tighten the clamp, spin, repeat.

I'm quite confident in my fabrication skills, but I'm quite certain I'd create both an undesired waist, and an out of round clamp if I were to take a rat tail file to a stem.