Fixed one of those boxes the other day...from what I remember the fin has a tab of some sort in the middle of the base that goes into a sort of "t-slot" if that makes sense.
Basically, its some one off chinese production box. I have never seen, nor was able to find that type of box/fin. I would suggest getting/trying a ProBox fin as a substitute (contact Greenlight or look online). That should have a similar base, but it will lack the center tab I mentioned above.
Also, if you are in a pinch I would just try and FCS fin.