I've always used an adapted screwdriver to access the slot on a Willis and Bates NRV but I've never been entirely happy with this solution so I decided to make myself a special tool. I saw someone use an old flat bit for the blade so having a bunch of duplicates I set about grinding the point off the tip. Having done that I needed a cross piece but all I had was some 12mm threaded bar but it had to do. I drilled a 7mm hole in the threaded bar, cleaned the surfaces, applied flux and soldered them together. Admittedly, solder wasn't the ideal choice but I figured it should hold ok Here's the result... I'll admit, it's not pretty but it does the trick!