My instinct would be to run rebar that's slightly shorter than the diameter of the stone mold and run two rebars through the foam 90 degrees to each other like an X. Assemble the mold with the rebar/ball in the middle and fill. It should stay nigh on center. For good measure three rebar pieces on an X-Y-Z axis would be ideal.
Now cutting the rebar will be a trick (I think it requires an angle grinder) if you don't buy it to the length you need. An alternative will be to buy small diameter rebar (say 1/4-1/2 inch) and do the same. Much easier to cut.
But I've never made an atlas stone so take my advice with a grain of salt.
Good luck on the next one.