Hopefully the top view rough sketch below should give an idea of what I'm currently thinking of. My slim spindle (the one with the brushless motor) fits into the 38mm hole (I've not bothered to sketch the clamp) in the green part.

The grey parts are 1/2" alloy plate, the blue parts are 2" X 2" X 1/4" alloy angle. The rails will move up and down, with the carriages fixed to the cross slide. I just need to work out where best to fit the 12mm x 3mm leadscrew.

I think I can probably trim the edges of the angle behind the spindle mounting plate, and mill a slot in the back of that plate, and "just" get enough room to run the leadscrew down the gap. Then I can fit a plate at the top and bottom of the slide, to take the stepper motor and leadscrew bearing respectively.

The sketch isn't too clear, but there is about a mm of clearance between the carriages and the angle that carries the rails.

Jeremy