Straightness of steel/alumunium box section

[Jonathan]

Ali is also lighter, dosnt rust (not like steel anyway) and dosnt need painting

True, but it's not as strong and more easily transmits vibrations since it's less dense...

Are you using the rails back to back on the y axis to counter the uneven load distribution? (ie 25% reduction at the open side)

Exactly.

if so have you looked at doing the same with the zaxis? if you mounted them on a c channel (or box with a slot cut out, then you would have space for the Drive screw as well)

I looked at it, but didn't think of using C-channel which would make it quite simple.
The place near me has some 3"x1"x0.25" aluminium angle. There's also U, 2" x 3" x 2" ... only 3/16" thick though. Time for some more drawing / calculation!