I had to do this recently and the advice from this forum was very helpful. If possible, try to secure the mushroom in place with a rubberbands. I was able to do this because the rock was small enough, and in a location in the tank where I could remove it. It only took about 2 days of this before it was set. As you mentioned, another option is to let it attach itself to sand and rock fragments, then using that as a base to epoxy it to the rock.