I used a razor blade and cut very close to the rock. It happened to end up with the tiniest bit of rock rubble still stuck on it so it made it easy to super glue but when you start touching them they get all slimy so its hard to handle them after a few minutes.
Can you not just put the rock they are on, on your rocks and give it time to fill out? My mushrooms are growing really fast so before long I wont be able to even see the chunk of rock anymore.