Eddy this is what I have been considering. Cost again could be an issue. A naive question, but I presume the starting point for accuracy is clamping the two top rail beams upside down to an accurate level reference point (precision benchtop) and welding top down (upside down)? I'd imagine working the other way will cause significant misalignment. Is this what everyone has done? I understand it will not get to micron accuracy, but it would aid initial accuracy before contemplating need for machining / epoxy?