if it hasn't floated away, you can go the tooth pick route. i just fragged several mushrooms this way. the best to use are the little cocktail swords. just stab the shroom near the center, and jab the sword into a crevice on a rock. within a week, you can remove the sword. worst case scenario, the sword doesn't hold, you can put a drop of crazy glue on the rock, then jam the sword in there. then instead of pulling the sword out, just cut it as close to the shroom as possible. don't worry about the shroom, it'll heal just fine.
hth