I usually just take a razor blade to mushrooms and cut as much of the disk off the rock then place it into a Tupperware with holes drilled in for flow but small enough that the mushroom chunks don’t blow around the tank. add some rock rubble and let it sit there while it heals. Once it heals it’ll attach itself to some of the rubble and you can then glue that rubble to a frag plug. Usually takes a couple of weeks to heal up. If there’s any of the foot left behind where you cut it from, it’s a good idea to put some superglue or epoxy putty over it if you don’t want a new mushroom growing back from the little bit of flesh left behind.
if you have big ones you can usually cut them in half with a razor blade and as long as a bit of mouth is on each chunk, they’ll both heal into their own shrooms.
I’m honestly not even that careful at all with fragging shrooms. They tend to heal easy and even small chunks of flesh often grow into their own shroom.