Hi Steve,

Joe's 4x4 is not quite 6x4 but gets alot of good reviews, and could possibly be scaled up. Still uses MDF, but also uses HDPE and bolted together 80x20 ali - might be worth a look to get some ideas:
http://www.joescnc.com/kitsplans.php

Or there's the Mechmate with the double gantry which has a good following. Mostly welded steel.
http://www.mechmate.com/