Temp sensor blew out of radiator

[JAGCHiker]

I have an '88 745 non-turbo that I installed a HD 3 core radiator last year. It has a push in temp sensor. I haven't had any problems with it; until today. I drove up to a nearby mountain and parked at 9,000'. When I came back to the car, the sensor had blown out of the rubber seal/gasket, with the resulting fluid loss. I pushed it back into place; with very little resistance. Although there was no leakage; after driving 50 miles back home- it was out of the seal again.
What can do I keep the sensor stay in the gasket/seal?