Hopefully this won't make you upset. But your Z-axis appears pretty thin. I think you are going to experience a significant amount of deflection. One suggestion is to replace that alum. plate (1/2 inch I guess) with steel flat bar, with at least one inch thickness. if weight is a concern perhaps Alum 7075 1 inch think would be a option. From the photos, I think you have about 2 inches of space between the Z-axis plate and the X-axis. You could also add square bar to the sides of the z-axis plate to increase rigidness (ie turn the plate into a channel. This would reduce deflection along the x-Axis direction of your z-axis.