Achieving flatness on a large gantry table

I just checked my table for flatness and was appalled. Max .025"
error over a foot at certain points. The gantry itself is within
reasonable tolerance. I have THK slides, machined extrusions and ball
screws. I checked it every step of the way and it looked great; until
mounting the table. This is a moving table design from Bob Campbell's
plans (a nice system). I first used standard extrusions clamped
together, I then went to slat wall, then tried to wedge together
machined t-slotted 6061 (not mic6). The errors occurred ar the flat
joins. With over 30 square feet, you certainly can't buy mic6. Even
if I could, I couldn't t-slot it. What would you suggest? I guess I
just expected too much. This system is good for plasma or router but
you certainly can't machine on it.