I never really liked the X-Carve base made of 20 x 20 mm aluminum extrusions and the MDF waste board. For me, it’s fun working with a CNC to really dial things in to make really accurate cuts, models and designs on the machine. I like improving the machine, and I compulsively over-engineer. My first Shapeoko was small, and then I upgraded piece-wise to the larger 1000 mm machine. The machine that turned into was on a frame of 2 x 4 studs, a sheet of plywood and two sheets of MDF. I then had MDF planks in between extrusions (used as t-slots) as a waste board. I had t-slots and the M5 inserts in the waste board. I moved, and I’ve been faced with setting up again. I wanted to make something that really is the best of all worlds in terms of being able to fasten my stock and work pieces. Here’s what I came up with using aluminum extrusions from 8020.net: an outside frame made of 40 x 40 mm, cross beams made of 40 x 20 mm and then shorter cross braces made from a bunch of old 20 x 20 mm that I from previous builds. It’s expensive to have the extrusions pre-cut and predrilled for fastening together. So. I’m holding it all together with t-nuts and braced gussets. The corners are being held together with 4 mm plates to get the initial frame very flat. My plan is to find a pretty flat reference surface to put this all together. Essentially, it’s an aluminum torsion box made of extrusions. Crazy? Probably! …but honest, I wouldn’t mind feedback. I have riser end plates and a new Z-slider as well, so the rest of the machine will be quite rigid.
No CNC can have too little rigidity 
Note that the strength of a torsion box lies in the top and bottom sheet + the ribs sole purpose is to create distance between the two layers so the weight is transferred to the opposite side, utilizing its tensile strength of the sheeting.
Do you think this needs sheeting on either side? Or do you think this will be adequately rigid? (It’s not visible, so let me note also that I also have aluminum plates on the bottom to reinforce the corners and to align the outer extrusions.)
Most likely it will be a considerable upgrade from stock, so by all means enjoy it
But to be a torsion box a top and bottom skin is required - and will be even more rigid.