actually I made a mistake in the slot width measurement of the isonic (I removed the caliper from the slot before reading, therefore I got smaller values!), the right value is more around 16.0/16.05 mm.
Anyway, I can only measure the beginning of the slot, the feeling is that deeper in the slot the thickness is getting smaller.
So I take even more happier your suggestion of leaving the fins as they are and work just a little bit, with sanding paper, the board slot.
By the way, what is the slot width you measure on the 2010 isonic 121 WC?