Well I've been playing around with it, and unfortunately I'm just not going to be able to get the work size I want to work in the space I need if going with the raised gantry type design, at the moment the work area is more important to me than absolute rigidity.

That said, taking on board that feedback I will be tweaking the design a little to take the 15mm mounting plates higher up (thus making the gantry arms effectively 40mm thick until they leave the bed surface) and I can also move the gantry down by about 14mm and make it line up with the bottom of the z-axis without losing any clearance... I can also switch from 20mm plate to 15mm plate for the main bed which would allow me to drop another 5mm off the gantry height. All adds up!

Given that the current machine does a pretty decent job of aluminium already, I hope that the re-designed Z-axis and the significantly beefier gantry arms (current ones are 150mm wide and 15mm thick vs the new one at 250mm wide and 25mm thick will be more than sufficient for my needs while giving the larger work area I need.