Too hard to find, I reckon do it with slot boxes (also hard to find but not quite so bad, and you get some adjustment which will be important in an experiment board)
Or if really want slot, mould them yourself from the fin base. Lots of mould wax (Nugget or Kiwi shoe polish will work) and glass it up. Then when cured rip out the fins, sand the box you just made into a rectangle :)