If it's a big piece of LR, you can use a small piece of netting (ie. cut out from fish net), to hold the mushroom down onto the LR. After a few days or a week, the mushroom should stick itself onto the LR.
Alternatively, place small pieces of LR or some frag plugs into a breeder box or similar, put your mushroom in, locate the box in a low flow area. When the mushroom sticks to any of the LR/plugs, simple glue the LR/plug anywhere you want it to be.