Hi Greeny,

Looks good and should do what you want it to.

Yes, box would be stiffer but I think this will be plenty good enough as it is. I guess you could plate over the back of the C-section that runs across the top between the two sides if you have a bit of plate spare.

Also the Y axis will be limited in stiffness by the small L bracket which holds the AC bearing block - I would reinforce this or look at a stiffer way to mount it.