I have two cokin-type holders - the standard and the wide - but neither are actual cokin ones, so that might make a difference. (I think mine are kood.)
On the standard holder, the cap fits perfectly. One reviewer says there is a gap between the cap and the lens, but I don't think there is a discernable gap. If you put the cap in the nearest slot (the one that normal filters won't fit in, which is for the polarising filters) it fits leaving no gap at all (as far as I can tell).
So it is ideal for just leaving the filter holder on the lens permanently, using this as the lens cap.
With the wide holder, however, the cap doesn't go all the way down. That said, it is is still adequate (for a lens with a 67mm filter size anyway) and - presumably - may fit better on the official cokin version.