I use an open toe darning foot. It is metal.
I also use the plastic big foot one. I took a small round file from my DH tool box, slipped it through the hole and gently 'sanded' off some of the plastic, going around the opening. This made it wider. I keep a glass emery file in my sewing supply drawer. It is tapered on the end for getting into small spaces. I also have the nail files with the buffing side. This finishes off smoothing out the repairs I have done on the plastic feet. No sharp edges were left on the plastic. Very smooth. I then tested the needle widths on my machine to make sure the needle wouldn't hit the plastic. I haven't had a problem with the screw loosening up, or any other part of the big foot.