Yeah a castle nut will get you what you want.. or castellated nut or slotted nut will all see you right

Cheers

Yep Castellated nut and split pin

you could probably get away with just a thinner nut and the split pin. It's only stopping it from completely un doing and falling off

Nails take the place of split pins for me, once the original split pins become unusable.....but I'm not suggesting you do that!

Actually, despite the reasons listed below, I find it preferable in some ways, 'cos I cut and bend them into more or less a circle, so there are no sharp ends to avoid when cleaning the bike!!