Ebay has been a prime shopping area and has yielded Bosch aluminium offcuts, stepper motors, linear rails and other bits and bobs. When all of this was cast upon the bench some dimensions started to appear. The rails dictated the x axiz length (I didn't fancy cutting them and I didn't have any longer ones) at 575mm and the ballscrew assembly meant a moving table was the style that would make maximum use of both items. A width of 300mm was chosen because its a nice round figure.
So far I've:
1) milled the top of the 45 x 90 extrusions to accept the 12mm linear rails
2) drilled and tapped 46 off 3mm holes in 3/4 x 1/8 brass bar to act as long "T" nuts inside the extrusions for the rails
3) machined two aluminium end plates at 300mm x 100mm x 15mm
4) threaded the ends of four 10mm bright mild steel bars which run through the extrusions
5) fabricated a saddle plate that holds the ballscrew fixed bearing / motor mount.
6) bolted it all together to check for gremlins.
I set the whole thing up on the table of the miller and put a clock across the rails and was quite pleased to find .05mm variation on parallelism.
Just for fun I hooked up the motor to the controller and ran it back and forth a few times with no nasty noises. I must get on and make the sub table next