You mean like this? I thought about that, but how do I make the red mdf part sturdy so that it doesn't flex upward from the weight of the gantry? Any flex at that point would mean the bearings don't make good contact with the rails.

I will be using 10mm bearings for the lead screw, one at each end. The total length is only 700mm, so its not a 8x4' machine that needs a massive leadscrew, surely? No?