Get yourself two one gallon nursery pots and cut the bottom out of one of them. Fill the pot with the bottom still attached with good potting soil and set the other pot with no bottom on top of it and fill with coarse sand then plant your ficus in the sand. Give it plenty of sun, warmth and water and it should take off. This will force the roots to grow straight down through the sand [ with little branching ] into the more moisture retentive soil below.

It shouldn't take too long. In a year or two you should have plenty of long roots to wrap around the stone.

Tony