The base of the entire system must be rigid (no bending or flexing), but I didn't want it to weigh a ton, nor did I wish to use steel. So I went with an old furniture makers trick - a torsion box. If you are interested in the details, check out this really great and quite entertaining site: thewoodwhisperer.com. Mark shows the construction and benefits of a torsion box.
First, let's look at a drawing of the complete base. Here is a front, side and bottom view. This will be helpful in explaining the construction of the torsion box. I did mine differently than Mark did on thewoodwisperer, but the idea and results are the same.
Cutting diagram. I went with cutting notches in all the cross braces so the framework slipped together snugly - I added glue to all the joints and used wood screws around the perimeter.
It is as solid as a rock. There will be some trimming necessary for the base to make sure of a solid fit against the bearings on the X-Axis assembly, so you might make it a bit bigger than the 22 21/64" that I had so you can trim off bits until the fit inside the X-Axis assembly is perfect.
Since the assembly of the torsion box may not be too clear, here is an exploded assembly drawing.
Here's an underside-shot of the base. I am sorry to say I don't have any 'under construction' images.