That plug is just a little cup which is held in by pressure from the cam cover. The cup I saw had a thin rubber coating which is what probably softens and allows the plug to be blown out if the crankcase pressure gets too high. A little RTV or other gasket sealer would be a good idea as far as I can see.

The plug in my son's '88 240 blew out while he and his wife were on the way home. He said it made quite a noise and sprayed oil everywhere. He was able to get home but only had a couple of miles to go. He and I replaced the plug and found he had let the flame trap plug up. I fabricated the strap and installed it in my two 240s just to have insurance the plug can't blow out. For the little cost and time, I think it is worth it. If you don't have the tools to make one, buy the IPD version.

When I have the opportunity (cam cover off) I reseal mine with some of the old time black gasket cement. I don't use a mechanical device to hold them in but if I did I would use a self fabricated device such as the one Don Foster offers up in the image below. Inexpensive (almost free) and surely effective.

Randy