The piece of plastic at the back, next to the record button, unglued

The piece of plastic at the back, next to the record button, unglued

This weekend I was in South Florida. It wasn't particularly hot or humid, but suddenly I noticed that the thumb rest - on the panel on the back of the camera, next to the red cord button, had come unglued. I found the piece on the ground, and it was stick enough to put back on. However, it isn't stable. How can I repair this?