Geberit Valve Nut Driver

While attempting to fix my dripping toilet water tank, I came across this corner valve that I was supposed to close, but it wouldn't budge since it had not been moved for 20 years. Due to some genius engineering on the manufacturer side, it's placed real close to the side wall so that you can't really grip it. It "offers" something like a slit on the top that looks like you could operate it with a big flat head screw driver, but since the knob is made of plastic, it only ruins the cap. It sure would have worked with a pipe wrench, but would only have ruined it even more.