I've created a repository with step-by-step instructions for printing a stand-alone dual-spool filament holder. It features four ball bearings, glides smoothly, has a quick-release feature, has a more intuitive filament path to the run-out sensor block and even protects against cross-over and filament sticking (typical of Shaxon carbon fiber, for example, and evidenced by a drawn-up spool*). I've used this technique for three months now and haven't seen a single job outage related to cross-over. There is a kilogram version and a half-kilogram option as well. I will later incorporate a dry box for this since I'll be printing with PVA. * In one case, it is guessed that this condition led to a blown fuse on the Robo board.