Thanks for the data. A 0.1mm deflection is too much for what I need. I'll support the rods at the bottom in some way and make sure the supports can't deflect sideways either.
I'll check the non ferro company, where I get the rods, what kind of profiles they have I can use, otherwise I'll have to use MDF. As an RC model builder I like to find a smart way instead of an expensive one.
I've got a pretty clear idea on how to proceed now. I plan to put the whole development of this machine on my site at
http://www.dream-models.com so others can copy it or give comments for improvements.