http://shopping.rcmodel.hk/advanced_search_result.php?keywords=diff+screw &osCsid=qi8pgu7sbdt6l4h0a9oq09gis0
I ordered the Hot Bodies and HPI screw. Today I tried the one of Hot Bodies, and it is a nice, very tight fit, the small hex is really stuck in the small black part with the 2 pins on the outsides, which makes it possible more reliable. I wasn't able to drive the car just yet, but it seems to be a lot better now. But of course: don't forget the locktite to be sure.