Now before I ruffle any feathers, this is a stop gap solution. This method will not produce perfect results, but it does work well enough to get you out of a bind. Remember to use safety equipment and don't mess around with tools you are not confident with.
Nut files are expensive and for someone who will only ever need them on a rare occasion, something I have hesitated about getting ever since I finished my first kit. Six months ago took the plunge and ordered a 70 odd dollar set that never arrived. So, I wanted an alternative.
First thing you need is a set of feeler gauges. If you don't have these already, $12 from supercheap, very useful for setting up your guitar.
I have a spare set so I wasn't too worried if this went south.
Find the feelers that correspond to your gauge of string
You need imperial measurements to match up with what's on the package.
Now you need a cutting or thin grinding disc in your rotary tool.
I used a 1/2mm wide diamond grinding disc, which was perfect.
Clamp that mother down.
Do not try to hold one in each hand, you will hurt yourself. Seriously.
Now, cut a series of grooves down the edge of the feeler.
No need to go very far, I only did about 2cm. Try to keep your cuts level and even, this will make it easier to use.
Depending on your set of feelers, you might have to go a fraction over size for one or two strings and you will probably have to use two feelers together for the low e string (cut them at the same time). My steel rule turned out to be the exact size I needed so...
Hooray! One set of feeler gauge nut files. I use a file to smooth out the edges and what do you know, still work fine as feelers too.
Now you're ready to cut your nut!