To make a triangle head screwdriver, you will need:
- Old screwdriver
- Bench grinder
For mine, I opted to go with regular two-penny nails instead of an old screwdriver. Cheap, plentiful, and no big deal if I mess up. I recommend you make one with a nail before you put the grinder to your screwdriver, to get the hang of using the grinder if you're not used to it, and to make sure you know how to shape it properly.
If you don't have an angle grinder, you could use wet concrete, a la Tim Anderson's hunting knife. It might take a year or three to do it that way though.