I have used gelled super glue available at the hardware store with okay results. It is safe to use as that is what is in the more expensive aquarium trade glues. I have used it for years. I dry the rock as best as possible, make a small button of super glue on the rock and a smaller one on the back center of the mushroom. It has to be the gelled variety of super glue or this wont work. Let set a few seconds then, underwater place the two beads together and gently squash. If you don't move anything for a few minutes, you will probably get a bond. You have to be really gentle, but piercing the mushroom doesn't work. Best way is to confine the mushroom and the rock in a place where the process might happen naturally.