Going back to your sketch in post#19, I would extend the side pieces forward so it matches up along the whole side of the X bearing plate. You can then pickup another bolting point on the front X bearing between the 4off vertical bolts for the X bearing.

This side piece extension would be triangular in shape and fill in from the top of the gantry down to the end of the X bearing plate.

It would then (optionally) be triangular back out to the ballnut mounting point, or just go straight back.

This would provide much more support for the gantry onto the bearings for not much effort (apart from a bit more material).