A/C vent cover hanging from ceiling

One corner of my A/C vent cover is hanging down inside the MH because the screw in that corner was screwed only into the spongy ceiling material, not into any wood. I've probed about 2 1/2" into the hole with a small pick and feel nothing to screw into. I'm afraid to probe around side to side for fear of making the hole too big.

Any suggestions for how to anchor that one screw in the spongy ceiling? Or alternatives?

Thanks for any suggestions!