Over the years I have bought a lot of screwdrivers for this. I ended
up just making one. Usually, even if they are wide enough, the blade
is not thick enough and it still tries to cam out, screwing up the
plug.

You have a point.

When I got my new Merc and it was still in the driveway I took
one of those big screwdrivers with a fairly short shank and made it a
perfect fit with my grinder.

Unfortunately, I don't have a grinder, and I am not about to get one
just for making a screwdriver. ...may be I can get one to sharpen the
blade for my lawn mower...

Grind the end down to the point where it completely fiulls the slot.

It sounds like the type of screwdriver that people use for firearm,
like the screwdriver for setting a scope on a rifle. May be I should
look for one in a gun shop ... but I doubt they have one for 1/2" wide
screw. Seem like I am still better off getting the one from Harbor
Freight, and this is what I am going to do.

Jay Chan