Gonna bump this up again for public knowledge- Can confirm that the ares rail works if you go at it with a file, and stays pretty sturdy. The stormwerkz one looks to be a little more sane of a procedure though. Less potentially destructive guesswork involved
Your other option would be to order an ak5c upper and top rail straight from g&g. The rail attaches direct to the receiver and keeps a lower profile, and should fit onto your fnc body just fine.