I've made tools for them from large blade screwdrivers. Usually cast off screwdrivers that were not much use for gun work in general but of good old school quality, so they didn't get thrown away.
SMall size flat files that are worn out work well too.
The files need to be drawn back as they are too hard to work on.
File the blade to shape leaving two 'pins' in place so they engage the holes in the screw head.
A good quality screw driver will have a Spring Temper hardness already. So it is easy to file with a good quality & SHARP file.
With all that said,,you can try the Bubba method of unscrewing it with the idea of making it into a regular slotted screw head once it's out of the gun.
The 'slot' would just run across the head and be wide enough to erase the existing holes and deep enough to be able to safely engage a screw driver blade for easy TD of the rifle when needed.
A Hacksaw blade with the kerf ground or stoned off of the blade cuts a nice screw slot for most American gun screws like this. Not having to go very deep makes not having the clearance of the kerf cut in the teeth a non issue.
The Bubba Tool which actually works on screws that are NOT stubborn is the use of two pins, one in each hole of course.
As close fitting as possible, the shaft end of common drill bits make excellent pins as they are tough but soft and will not break easily as the twist drill end.
Set a pin into each hole. Then take a steel bar w/flat sides and lay it in betw the two pins. Lay it down, all the way down against the screw head.
This one is domed so the bar will be pushing on each pin just above the screw head itself.
Now, clamp the bar to each pin. There isn't much room, but small clamps or even the small ViseGrips can be used to clamp the bar to each pin.
Clamping will keep the pins straight up and in place, but they must the tight.
Make sure the Bar is set up to be pushing against each of the pins as you will be turning the bar counter-clock-wise to unscrew the TD screw.
You have about as solid a set up as possible as long as you push straight down as you carefully Unscrew the TD screw using the bar as a T-Handle
...Ben
