I used plain old super glue, but the rock should be dry. Tried it wet (underwater) and came back later and the shroom was gone. Since the rocks were too big to remove and dry, I took to taking small dry pcs of rock and gluing to those (did it w/zoos too), then wedging the small rock in the larger rock holes. Worked better for me.
Otherwise I believe you can also use bridal veil fabric to fix the shroom to the rock and wait for it to take on its own. Hopefully you can get shrooms
already affixed to a small rock pcs.
I'm sure others will come along with better solutions, but those worked for me.