First off, this is a remix based largely on the work from ckitching (https://www.thingiverse.com/thing:2923937)

The idea here was to add a filament length indicator to the design so it is easy to see how many meters of filament is remaining on the spool.

If you already have some of ckitching's alternate spools on hand, it is possible to add the filament gauge by printing Spool - Indictor.stl. The gauge is designed to fit flush with the inner wall so it won't snag the filament, use a bit of glue to hold it in place.

Otherwise, print Spool - Inner.stl and Spool - Outer.stl. This spool will fit EconoFil refill filament from filaments.ca