Step 1: The Set-Up
Now I had thought the easiest would be to just make a fitting flat-head driver and try that, but it didn't work. In the end it was quickest to just make a complete 5-wing Torx.
I didn't take pictures back then, and the original remained with my friend, so I wanted to see if that was just dumb luck, or if the result could be recreated. I got some gadgets from a Flea market today, among which an Ipaq with dead battery, and a bluetooth gps dongle. These have some screws which are slightly smaller than the smallest bits I've got in my toolbox, so I gave it a try. So
(BTW, the camera is from the same flea market, so I'm trying that out at the same time... Excuse the image quality.)
Step 2: Ingredients
-A File: A triangular, fine metal file should do the trick
-Wire: I used a clothes hanger because I happened to have it, but it proved to be a good compromise in strength while still being able to file it. Copper is too soft, piano wire is not easily filed.
-Pliers: Give you a better grip. If you have plier-like hands, you might skip these. Freak.
Step 3: The blank
Step 4: Start filing the faces
If you're confident, and actually bothered to measure, you could already make this the final thickness between two faces. I eyeballed it and rather left a bit more flesh, then to end up too thin.
Next I file perpendicular to the first to faces, to make a rectangle. These temporary faces contain the edges of the remaining faces. Again, one could measure the distance between two corners to determine the distance between these faces. Again, I eyeballed.
To make the edges visible in the following step, I color the small faces with permanent marker.
Step 5: File remaining faces
If all went well, this should result in a Hex key of sorts. I go around all the sides until they are even, and hope that it is still thick enough, when they are. If you're making an Allen key, file down all sides until it fits the screws.
In the next step, we'll start from this to make a Torx. For this the Hex shape can still be left a bit too large.
Step 6: File the Torx
Getting it to fit the screws takes a bit of patience. At least Torx have some redundancy, if some of the wings are less pronounced, it will probably still work.
Step 7: Results
All in all, the Torx is probably more suited to this method. Even though it appears more difficult, it is actually more forgiving in performance, once it fits. For the Allen key, the margin between fitting in, and slipping is very small. The soft metal used here also makes it easy to over-tighten the screws, damaging the screw driver. If that happens you have to file the damaged area away... An upside to this is that it is nearly impossible to damage the screws themselves. A big plus when trying to remove recessed screws.
Below you can see that the screwdrivers fit well enough for the screws to stick, just from the friction.
This completes my first instructable, I hope it will be of some use.