If you're talking about that middle one, it was recommended to me to double nut it on the back side. More recently I saw somewhere online where there is a square nut that will fit snug in that area and won't turn. I found them at Fastenal but they wouldn't sell them to me, haven't checked Ace or Atwoods yet.
It's a common thing and I have the same problem so I was going to try one of the above cheap fixes along with some lock tight.
FYI - one other thing I heard of that's more permanent but avoids a helicoil is getting a stud with two different sizes (drawing a blank on what they're called...?). Seems like it's something like M8/M10? You tap the stripped one bigger then you can keep the original cap nut on the front. But since you already have a fancy new stud, you probably wouldn't want to bother with that.