I've been toying with the idea of using supported 16mm rail for a Z axis, but was thinking of fitting the rails back-to-back, with the rails facing outwards. The idea is similar to the angled version already shown, but would be easier to fabricate from standard parts. The Z carriage would be channel shaped, to keep the linear bearings loaded in the right direction.

I had thought about doing it the other way around, too, with the rails facing inwards and a block carriage sliding up between them. This ends up a bit wider though, so might limit travel in the Y axis.

Jeremy