I am working right now on a large platform printer as well. I am working toward a 16"(400mm) x 16"(400mm) x 24"(600mm) build platform. I am curious haw large a build platform you have chosen? I am concerned that the v-extrusion 20cm x 20cm may not give enough rigidity to the frame to handle the quick back-and-forth motions without wobbling the print. I also plan to use direct drive instead of belts.