Screwdriver bits for WIDE screw heads

The stubby coin slot driver's not going to help with the screw in the OP unless you want to mess around with a $26. screwdriver.
Back to go do not collect $200.

The maker of the coin slot screwdriver I linked to, has at least one more size, which looked better to me so far as fitting 35 SLR's coin slots.

Some people actually enjoy using good tools, and these screwdriver/slotted cover removers will be enjoyed by more than a few photographers that like quality tools.

IMO.

I've used a hacksaw blade, grind off the teeth and grind it to a nice, square tip. The thickness serves well, and the steel is pretty hard.

The screw in question is not a “coin slot” screw. The slit in the screw head is flat, it does not have a rounded bottom. Anyways, I bought one of the “Leupold windage” screwdriver bits, which is too thick but more than wide enough. I’ll grind it down carefully and it should do just fine. The bit cost $6 plus shipping - I bought some other stuff, so shipping was pretty minimal.

Careful with the grinder. There's no reason that won't work. FWIW get closeish with the grinder and finish with whetstones.

That's a lot of material to grind away and keep the geometry sweet, especially in light of the actual slot size, before the mangled bits widened it's appearance.