Hi
Fileing a square hole is a steel boss probably isn't as bad as you might think. The key is GOOD Sharp (read new) files. Old dull files are frustrating at best.
This is a great exercise in craftsmanship. First, with a new file, cut on only the push stroke, then lift and repeat. after a few strokes remove the file from the work and brush it clean with your hand or better yet a file card following the direction of the teeth. then proceed patiently and carefully. with good tools and some practice you can file way more than you ever thought you could.
Jonny