Reduce 8mm square tube to 7mm square tube without pro metal tools?
I'll disagree with (most of) the comments above and say just file it. You are removing a small amount of material, you don't want to remove too much, and it won't take very long to do each one.
Without crafting some sort of jig you're far more likely to overdo it on at least some of the handles with a powered approach, and/or to make the result un-square.
I don't think you are doing so many that such a jig will be worth your time to build.