I built the Y beam and test fitted it with a piece of scrap for the Z carriage. It is really stiff yet slides with ease. All 4 bearings have eccentrics installed for adjustment.
I used some old pieces for the internal support web, just to add some memories to the evolution of this build.
There are holes drilled though for threaded rod to attach to the gantry walls. I also plan to add dowels after final placement.