I just Gutted an old office chair and its just made up of 4 bits of 1-2mm angle that was floating around the factory 2x450mm for the cross bars 2x300mm for the front to back runners The recaro pitchings are 405mm apart And using the front hole as a datum the 2nd hole is 25mm back and the rear one is 260mm from the front one. (Google a diagram for the pitchings) You could easily just use some 30x5mm flat bar or something similar if you dont have angle floating about