one thing that does spring to mind:
if you lay up your board similar to a strip-built boat.
you will find that shaping the joining edges with matching cove and bead will help to maintain the desired bottom concave. But i doubt it will do anything to hold the rocker shape. i suspect all of the rocker will go away once the core is removed from your frame (rocker table for the rest of us).
if so , you may wish to
consider steaming the core,
laminate the core with 2 thin layers of wood. (top to bottom)
or ....glass as you originally intended to maintain rocker.
but unless you'd like your board to be very stiff, (which is not so terrible) less glass and/or less core may be needed.
just a thought ,