Futures Box Fillers eliminate the holes left behind when you're not using all your Fin boxes. (i.e. a 5 fin) Using Box Fillers is a no brainier, you'll reduce drag and make your board more lively. If you have a five fin setup on a board that uses Future Fins, then the Future Fins Box Fillers area must.
Futures Fin Box Fillers: available in 1/2 inch, and 3/4 inch