If I was to build an x axis out of aluminum extrusion, say a 40x40mm profile like this one, would filling that hole in the middle with concrete be an effective way of adding weight and stiffness to it? I know a lot proposals for concrete cnc machines have been put out, with the main issue being shrinkage, but I think that since the actual mounting would be done on aluminum extrusion shrinkage wouldn't be an issue here.
Also beyond this, what about making a truss out of PVC/gas pipe and filling it with concrete?