Jcb's CNC machine Mark 2.

[EddyCurrent]

You might get a granite plate at Tilgear for reasonable money.
Your design makes for a very rigid gantry and it highlights something I've been thinking about. Say you wanted a height under the gantry of 200mm then with your design you would raise the Y assembly from the bed by about 200mm but then how do you make the 200mm risers stiff ? probably by using a welded steel frame ?

Edit: I should have checked out more of the build logs first, there's quite a few along the same principle