I wanted to pop a spool into an EIBOS Easdry filament dryer, but the cores from the original were too thick to fit the peg they needed to rotate on. The wall thickness was 4.5mm, which seems far thicker than this really needs. Since the insert is flush against the spool it doesn't carry weight. So I slimmed the wall down to 2mm, which should be more than sufficient.

TODO: It is a bit tight going over the rubber band on the dryer core, consider 1.6mm wall thickness to gain a bit of core diameter.
TODO: The 9-pointed polygon makes the points of the face less prominent. Maybe add 1mm. Or just go with 7-pointed like the original.
TODO: Chamfer the face to give less to catch on.
TODO: Think on whether the walls need to be that tall. The original is 10mm, but 5mm is probably sufficient.