Just normal insulating foam from Home Depot works great. I had a bolt in my Jeep that fell into the frame rail from the top and I tried for an hour to get it out through numerous holes. Just could not get it out through any of the holes. I went to Home Depot, purchased the foam, with a magnet got the screw right to the hole, foamed it to death, and the rattle was gone.
You can also insert another screw through the hole to hold something in place.
To be honest, it was not my idea, but a union tech I had in TN who worked for me. He was the best tech I have ever met, and when he retired, we lost one of the best union people I have ever met. He had one of the best work ethics of anyone.