In looking at the ones i pulled out of my F2 head, it looks like the nipples were not part of the original design, but the spacer was taller than the fin space, and the nipples were 'formed' by the pressure of the spacer between the fins. ICBW 
Anyway, for the new spacer design, the nipple height on each side will need to be 1/2 the width of the fin or they wont all stack correctly (as the one above and the one below must share the nipple hole space)
Franken - i'd be in for a set if you do grey/black.
Having just bought the OEMs, the nipples are part of the rubber insert by intention not by pressure.
Also, the rubbers go in every other fin, skip a fin. So their length is not critical. Except in the case of the bottom 2 cyl fins where they were consecutive.
However, the nipple on the OEM was smaller than 1/2 the thickness of a fin, so it's not an issue with OEM.
My comments are for a K2. Other models may vary. (F2?) Though the quantity "20" was in every fish I've seen on Ks. It would take more than 20 to put them in every fin without skipping.
