Second CNC table build, here we go again....

[m_c]

Just done a rough sketch of what I was thinking.
.
If you wanted to add support to the side rail, you could use a wider box section, and either move the rail nearer the edge and have the legs/supports mounted on the opposite edge, or use a double box section, with the rail on one and legs/support on the other.
.
Dust could be mostly eliminated by adding a couple brush strips, or suitable strips of metal with minimal gap either side of the rail to further reduce the ability of dust to settle onto rails.
.
And having just looked at my sketch and thought about how to protect the screw from dust, mounting it at the side of the rail so you only have one 'area' you need to minimise dust ingress might be an option.