Single FCS slot

My son got a board with normal 3 fin setup, but on the outside and slightly up from the outside fins, there is a single FCS slot on both sides. I have looked for a fin that fits this, but haven’t found any. Is there a nubster for the outsides?

Any ideas?

Maybe could take some cheap FCS fins and cut down?  ideas?

 

Thanks

Luke

C5 Setup

http://www.theshapingshack.com/c5-box-and-fcs-fin-set-2/