ok, first attempt using sketchup 8

[deannos]

Adding several of the green triangular pieces might help, but it's a lot of effort to cut them out without a guillotine.

I have a very nice angle grinder that would do the job.

Other option is use box section but add plates on the top and bottom so that you can put the linear bearings off center to get the ballscrew in a good position.

You mentioned earlier about the ballscrew being in a good position, i'm not to sure in what you mean. If you mean to bring it out closer to the Z plate, could i not use spacers on the bearings