You've come up with almost exactly the same design as I did, even down to the materials you've selected. I've switched back to using extrusion though as it's simpler to mount to the gantry sides and I couldn't think of a good way to bolt the two pieces of box section together.

One thing I would add to the comments already made is, depending on the travel of your Z-axis, you might want to make the front plate a bit taller to give yourself a better spacing between you Z-axis bearings. Once you've switched the joints around it's easy to make the change just for the fairly small cost of the metal.