I've made the Y axis slider top and bottom faces of the cover symmetrical with the y axis tube. Not sure if it's added any visual appeal, but it has gained me 17mm of Z height. I now have an estimated 428mm from an 838mm distance from the bottom of the lower frame tubes to the top of the higher ones. This means I may achieve a 390x640x390mm build volume with the supervolcano melt extender.

I've also increased the distances of the Z beams from 750mm to 810mm now that the bowden tubes will be routed differently.

Now, I'm wondering if the 27mm tubes are the weakest link in this. It's still a wider diameter than what's used on the MPCNC (and probably has thicker walls) so I should probably try it before I deny it.