This is not a good idea

BWalnut said..
Most likely you could slap two center fins in the track boxes but I've never tried this. However, I would not spend ANY money attempting this. The shape of the foilboard rails are not designed to carve on waves and the placement of most modern boxes is not even possible for carving on waves. I think your best case scenario is that you would be able to catch and ride the whitewash straight back to the beach.

Love it! Heading out to Mav's tomorrow with my downwinder, I mean big wave twin!

If I was to try it. I would get an old mast cut short & cut to the shape of a fin.
It just depends on what type of surfing you want.
The fin set further forward will make it looser.
Imagine how it would go having a mini wing to be able to get the board to just leave the water surface.
In the early days of foiling I converted my 7 foot SUP that had an excessive nose rocker & some tail rocker into a foil board. It was slow to paddle because the nose rocker would push water & not glide over or carve through it. Even on a wave it was slow unless you could pump it like a normal performance board.
But with a foil the nose would leave the water & the board flew as it picked up speed when catching waves. I still use it now for foil surfing.