Steel is cheaper, faster and easier if you don't have a mill or other cnc. 10mm steel and that having in mind the way i did it, where the rails and the rectangular bars that the rails lay on, play the role of ribs that strengthen all. You will be surprised how flexible is 10mm of steel if not strengthened by bracing plates that provide strength in contrary direction.

I managed to mill with that Z/180mm fully extended/an aluminum plate- 1.5mm deep pass, and lack of cooling and proper bed fixture was the problem to go deeper per pass.