How about a rivet nut insert?
https://www.amazon.com/dp/B072QWZPWK
Or possibly a threaded nut insert? You can tighten the insert down, clamping it onto the heater box. Add in some loctite or epoxy and it should never come out while backing out a screw.
https://www.amazon.com/dp/B00YJDZ2JS