I'm not sure if the angle would be right on a file but may work if the angle is right , I was thinking of a grinding stone as you can shape it to any size and angle or possibly an engraving tool , I think the seats are cut at 60 degrees. It shouldn't too difficult to make or get something the right size ,the cut would have to be very small , even to machine up a metal shape the right size in the lathe and use valve grinding paste would work .
