[Bren]

It's best to get one of the tools made for the Glock front sight but, if you can't, then you need a very thin-walled nut driver or socket - something like AVMan is talking about.

To solve the problem of the socket being very deep and the sight screw falling into it, just fill it with something - candle wax, cotton, etc. - as long as you hold the driver and slide right, you could fill it with sand, but a few drips of candle wax to almost fill the socket would also help hold the screw in place while you screw it in.