Not necessarily. I have closed ended ones. I just checked the overall length before ordering. From memory I think they need to be less than 20/21mm.
EDIT: I think the 20mm I had in mind must have been from the top of the radius seat. I think these are the ones I have...
http://www.ebay.co.uk/itm/20-x-RADIUS-A ... 53f1d33b30
They are 28mm long, which is 7mm less than my original nuts. The cover does touch the top of the nuts, but still clips in place. its an option if your'e stuggling.