Wow, thanks to irving2008 and Lee for the suggestions. I'm hoping to cut aluminium with this thing when its complete.

I am using 45mm cross section as I blagged a load of it from work, but I prefer the idea of bolstering the Y axis up with a 15mm plate between the two risers.

I'm committed to the sbr20 bearings and 20mm lead screws as these were donated as well.

I really appreciate the comments, firing up the CAD system as I type.
Tony