Thanks Eddy. I thought my design was too simple. Now I know why! Thanks for the link. It was extremely helpful in explaining COG and design considerations. Conceptually, I will look to draw the shape of a RHS section at right angles to my gantry to move COG back towards Y axis rails.

I've noticed that a number of designs have placed Y axis screw on top of gantry (my design has it on side of gantry between rails). Placing it on top would help marginally with moving COG back. Is there an inherent preference for placing screw on top? Is my design inferior or not practical?