Hi Charlie,

That closer plate would be very expensive, especially aluminium. Is the idea that the ply panel would hang out through the gap?

The problem with that approach would be making sure that the rails mounted on top of the long extrusions did not see any load / deformation when the closer plate was added. I think if you put a DTI onto them and mounted / demounted the closer plate the rail would move as you tighten the bolts. Best to get it all rigid, using epoxy levelling if desired, then leave it alone.

The other issue would be the stiffness in the lateral direction (i.e. towards and away from the other long side) would still be low, even with the closer plate. Usually this is helped with supporting posts which have some section in the Y direction - for example short extrusions of the same dimension as the long extrusion but turned on their end . . .or if done in steel then box sections turned on their end.

Google 'Joes 4x4 hybrid' (cnc zone) for machines in landscape format rather then portrait format - not saying I'd go with a machine like that as I'd say some of the features are a bit out of date now but worth a look over, plus the gantry gets quite wide.