i have only made the z axis
here's what i used. 6 inch square box tubing with .5 inch wall 36 inches long 1 inch plate at the bottom. put some .5 plates front and rear.
most of my cutting is done with in 6 inches of the table. the rest of the z axis seems to be used for changing tools.
just double up your z axis to make it 8"x8" and that should be ridged enough