Sides
The high gantry sides introduce a weakness - delete these, raise the longest X axis rails (and supporting aluminium profile beam that it is sitting on) so that they are level with the lowest point on the gantry cross beams. Then re-draw a small gantry end design that links the gantry beams to the X axis rail bearings. This is much stiffer overall as the raised part is now integrated to the bed and can be supported as much as you like.
This would also mean you could delete the EXTRA lower pair of hiwin rails as there is virtually no moment on the gantry ends anymore. This would save alot of money!

Gantry beams
Putting rails on the inside faces of the gantry cross beams makes it difficult to get them aligned/in the same plane. You can't use epoxy leveling for example, and those aluminium beams are not flat. This means you will need to use shims etc. which would be fiddly to get all 4 aligned. Machining them flat to start with might help, but they need to be accurate to prevent binding. These precision guideways do not like much mis-alignment.

Z axis
You have not shown details of the 'box Z design'. Be aware you need to allow for head tramming adjustment, Z rail alignment, and Z rail pre-load /adjustment in the way it all goes together. CAD is 100% perfect, but real world is not.

As per Deans comment if I was starting out I would not go for something quite so complicated.