hi ross
even with the centre section cut out there is still no flex in the beam, also reduced the weight , it was very heavy... wall section is 6.5mm.
was intending to put a few 8mm threaded rods through from top to bottom , along the lenght, with 2 nuts inside and and 2 outside which would have held it rigid, also could have adjusted the preload on the bearings (as a adjustable spacer) but think its going to be ok. can always be added later

with the screw inside keeps the y axis and z axis very close , going to cut the centre section out of the z as well , to keep router as close to the y axis , to reduce cantilever

holidays start tomorrow so plenty time to rebuild

what type of rail are you going to use?