That's a fairly common clipping issue with the game, but unfortunately there's not really a known fix (at least, that I know of). You could try lining the sides with thin invisible FOFs, but that might just end up looking weird in-game.
Just out of curiosity, though, is each sector with the FOF in it a single contiguous shape? If not, it may help to do that, and maybe divide each sector into convex shapes? I dunno if either of those would help (and unless there's some geometry outside your screenshot, you're probably even doing both of those things already), but it ...might... do something?