This is my first BUILD please help.

[DigiSoft]

Its always better to have the y-rails as far apart as you can and as low down to the bed as you can to improve torsional stiffness...

I think you could reduce the thickness to 12 or 15mm without undue effect...

What is the box section between the rails? material and dimensions?

This is my newest version. I found that I must put the ballscrew in the sandwich so I can use the maximum length.
The box section of the two pieces that hold the rails is 50mm x 60mm at 4mm thickness?. I am thinking of using stainless still. What do you think. Will they bend?