I make all my gun - screwdrivers myself:

use any quality screwdriver
choose a driver that fills the length of the screw slot
if too wide, file to fit
place driver horizontally in a vise
file the front of the blade (about 1/16th deep)square
turn the driver around and do the same on the other side
turn and file until desired thickness is achieved

if you know how to use a file (two-handed) this will take
5 minutes and give you a 'bespoke' driver for that screw
(I use a 3/4" wide flat file for better handling)

regards from England
Gunter
NRA Life 1974