I thonk it should be one of the small captive nuts. They are housed in a spring clip that secures in the hole. Definatley not a well nut, but it should work ok?
Do we have a pic of the correct fitting?,got any in stock Neil?. Captions a strange one,there was a guy,in MCN I think,who does wheelies sitting on the headlight frame-I,ll leave him to it!.