Files and rasps usually come without handles, and it seems to be getting harder to find good quality ones at local stores. I prefer wooden handles, but have come to appreciate the feel of the grip on this handle:

It’s made from plastic, but I find the shape and size to be a perfect for my hand, so I used this as the template for making my own.

To get started, I roughed out a piece of clear spruce into a 1-1/4″ cylinder, long enough to make two handles. I figured that doing two handles at the same time would be more efficient and keep the stock short enough that it would not have to be supported:

A comparison to the handle I want to copy tells me how close to the shape I am. It could be more precisely laid out, with marks at the transitions and calipers to measure the diameter, but doing it by eye will get it close enough.

I continue to refine the shape until I’m happy with it, then part the handles off:

Quickly remounting each handle back in the lathe for the final sanding and to “tune” the fit of the copper ferrule using sandpaper.

The ferrule is made from part of a plumbing 90 degree fitting and should be a tight fit on the handle:

I used the 90 since I happened to have a few on hand. An end cap for 1/2″ copper pipe is also a very good choice, but you’ll have to drill through the copper on the end to insert the tang of the file. On the upside, the end cap would provide more support where the tang enters the handle.
1/2″ crimp rings (made from brass) for PEX plumbing pipe would also work well, especially if they were crimped tight after the file was put in the handle.

Cleaning up the end on the disk sander:

Super glue holds the ferrule in place on the handle.

Drilling the hole for the tang of the file is made easier by putting the end of the handle in the 3/4″ hole in my replaceable insert in the drill press table. This centres it in line with the bit, and I just hold the handle with my hand as I drill it:

The hole is drilled slightly under sized and, in this case, I elongated it for the tang on a flat file. The file should fit snuggly in the hole, driving it in by tapping the end of the handle on a hard surface. Glue should not be used to hold it in, since you will probably wear out the file before wearing out the handle, and you can reuse it on the replacement.

I made four new handles, two from maple and the two shown above, from spruce. After using these for a while, I would say the spruce handles feel a bit more comfortable, for some reason.
I made a simple rack from a piece of plywood to hold them in a convenient place (above my metal working table):