After taking in suggestions from forum members, here is my lastest gantry design. It would consist layers of 12" x 44" MDO plywood laminated together to end up with a beam that is 12" high about 3.5" thick. The bottom 3 inches would have the center sawed out of it, but turn down at the ends to meet the linear carriages. I'm thinking the sides (green) would be 3/8" aluminum plate. This design should be very resistant to any gantry side to side motion.

In the interest of time, I didn't put the details of the ball screw supports in yet.

Also, instead of sawing a notch in the beam for clearance for the X screw behind the spindle, I'm looking at putting the X ball screw on top of the gantry beam. My concern is pulling the Z axis along from above both X rails instead of between them. But numerous ball screw and R&P commercial and DIY routers have the axis driven from the top of the gantry beam. I'm looking for advice as to if this could prove to be problematic.

Thank you,
Jerry.