Thanks for the suggestions, I had considered using counter holes in box section but theres an awfull lot of holes, and I want to keep it simple and as quick as possible to build.

I did run some stress tests when I went for the channel, cant remember the results but it didnt deflect to much, I've no idea what sort of forces are involved, I just put a force of 500N in various directions at the tip of the router, I will re run them and post the results.

If the channel twists to much I have thought about closing the open end with some folded ally channel as a work around.

Thanks for the link and suggestion to the cable anti-racking system, I like the look of that, it appears straightforward to do and could be done afterwards if I get problems so I think I'll stick with the single center ballscrew and use something like this if it becomes a problem.

Vic