Splitting the gantry into two box sections as you have done is a great way to do it. I'd put an aluminium plate on the back to stiffen it further, although it's probably fine as it is.

The gantry sides just look flimsy. Since you don't have an adjustable height bed most of the time the Z-axis will be operating near maximum travel, so the combined tool deflection from Z-axis and gantry bending will significantly limit the feed-rate and finish you can obtain. You should do what I put in the last post - eliminate the gantry sides and have the box sections for Y on the smallest plate, or box section, viable to link them to the X-axis bearings. Add to that an adjustable heigh bed and you'll have a really strong machine with as much clearance as you want for long tools/drill etc.

You need to consider how to accurately align the X-axis profile rails. Currently you can get them both parallel in the X/Z plane, but the spacing of the rails (i.e. parallelism in the X/Y plane) is fixed. No frame will be accurate enough to allow that unless you build in some adjustment, or fix the rails in the same plane as the bed (like on mine).

Correct me if I'm wrong but you've drawn the rotating ballnut mounts fixed to one end of the bed, not fixed to the gantry sides?