Fizzy,
If it is any consolation, I made one for The WaterWorks Museum in Hereford. Bought a slab of marble from a stonemason in a local town.
Made up a sturdy angle iron frame, with internal cross bracing, to provide a flatt(ish) surface on which the marble could sit. Each corner had an adjustable levelling screw, at the bottom of a vertical piece of angle to prevent the marble from moving off the base.
Placed a sheet of hardboard on it, which was covered with wet, rather sloppy plaster / wall filler, before placing the marble slab onto it. In that way, the marble should be "floating" and free of stresses.
The 18" x 24" slab of marble cost me £30. Larger would have been too heavy to place / remove from, the boot of the car! You may be able to get a better deal.
Then made up, and varnished, a simple plywood cover for it.
Howard