I don't think the backplate on the rear of your z-axis adds a great deal to the performance of the machine, if you get rid of it you can bolt a plate to the back of you gantry to improve stiffness. If that still doesn't provide enough stiffness for what you are using the machine for you can always pour concrete into the box section.

Concrete is obviously heavy but weight can be reduced by adding things like vermiculite/perlite/polystyrene beads etc into the mix to displace the sand and and some fibre glass or fine steel fibres to add reinforcement.