A lot of folks put the gantry cross piece for the ballscrew underneath the square tubing that supports the linear rails instead of running it between the rails. This allows your table to be supported by cross pieces instead of just at the ends.
Another thought is whether you want those rails mounted on top of the square tubing or on the outer sides. Having it on the outer sides gives it a little more protection from the mayhem happening in the middle although it is theoretically a little weaker for the rail. It also makes the top edges very clear in case you need to drag heavy sheets of material over them and don't want to damage the material or your rails.
Now you can imagine the gantry being a hollow box that wraps all the way around your support frame, slides along rails in the middle. It looks a little less strange, you get to use a single uninterrupted piece on either side which is simpler, and the table is better supported.
Just a few thoughts for your consideration. Make sense?
Best,
BW