Are your shrooms totally free-floating as in not attached to anything? If they are on rubble, you can superglue (with gel) the rubble to the live rock.
If they are free-floating, you can put rubble into a shallow dish, and put the shooms on top of the rubble. You will then need netting of some sort. Secure this neting over the top of the dish so that the shrooms don't float away.
If you have intense lighting, put this dish somewhere shaded in the tank. It will probably take a week or two for the shroom(s) to attach to the rubble.