They do make a all steel lock nut that can be used over. It could be soldered to a tin or brass plate & that held to the rear of the firewall with small screws. The nylon insert would melt from heat if soldered .
Several aircraft I used to work on in my old AF days used a nylon nut held in a stamped alumimnum holder that was riveted to the ac frame with one or two rivets . Never did see any changed over the years but they were not in any high vibration areas ,mostly just a tough place to put a nut on . " Keep em flying " Max H.
