Driver Size for Euro Slotted Nuts

OK, I’m going to make my third attempt to buy the correct size driver for the slotted nuts for my DIY Euro modules, from here. Could anyone tell me definitively what size I need to get?
The M6 is Definitely too big, and the M2 too small. There’s quite a range in-between, though.

a|x

Do you mean these nuts ?
Then I think, none of the M2 to M6 slotted nut drivers will fit.
Have a look at the erthenvar specifications for dimensions.
They have also the right tool at erthenvar.

what nuts are you talking about? The erthenvar jacks & similar are definitely M6: http://www.thonk.co.uk/shop/jack-nuts-and-washers/

Eagle plastic devices makes one as well, you can get them at mouser. the quality is a bit weak (mine needed a little filing) but it has a center post that prevents it from slipping off