Thanks Sven, after Robin's comment about having a big chunk of ALU that can be sold if things go horribly wrong I have to admit that I'm rethinking things a bit...

I'm not too concerned about thermal expansion because I'm building a CNC machine in a garage that is attached to a house and the temperature doesn't won't really fluctuate more than 15C over the course of a year.
I was banking on the epoxy getting vibrated to the top and selected the epoxy especially to do that as it will self level.
The milled carbon amount is based on the best practice mix on the easycomposites website, 10% by weight compared to the epoxy. Obviously the quartz/granite/etc in the mix doesn't count, this results in a surprising small amount of milled carbon fibres that add greatly to stiffness and dimensional stability.
I'm not sure exactly where the optimum lies between tube and square, I selected square so that I could weld it together easily, circular tube would be much more difficult to fabricate.

I've been arguing the toss with a structural engineer and insurance company about the fact that my house has giant cracks in the extension so for the next couple of weeks/month I need to back burner the CNC which is very annoying, I am relying on it to do a lot house upgrades. :(