Even More Reliable Filament Runout Sensor and Spool Holder

This version is operatonally derived from my earlier "Reliable Filament Runout Sensor" (https://www.thingiverse.com/thing:2483919) from last year. Both versions are designed to mount on a 2020 bar which can be part of the printer or part of a separate filament holder. The first design works very reliably when filament runs out and the end of it comes free from the spool. BUT -- there are times when the filament is taped to or bent through an inaccessible hole in the spool so that it can't