There's many types of files, some having coarse teeth on the narrow edge of flat file.

I find Lennox blades to be the best by far. The shorter the stick-out the less flex in the blade as you work. Found at you local hardware, it's a small cast aluminum or plastic handle that blade slips into, handle surrounds it, with blade sticking out from end. Because the holes are fairly large as 1/2' you can use a fine-toothed hacksaw blade in a one-hand handle.

That can be pretty frustrating by lo-bucks methods.