3D printable filament spool

This is a 3D printable 1 kg (88 mm wide) filament spool. One of the design goals was a reasonable compromise between strength and low material use. It weighs 60 g.

Update 2020-05-19:

* Added better inner attachments for filament to be used to attach the filament before rolling it up.

* Added a smaller version (28 mm total, together with the female) of the male spool.

* Slightly increased inner hole to more easily slide it onto cores of spool holders.

* Adjusted holes to make it ea