Ring-shaped bobbin holder.

Completely customizable. Would like to add some sort mechanism to keep the bobbins in place (magnet in the bottom of the groove? some sort of ridge running along the top so the bobbins "snap in"?).

Pictures show a print with an inner radius of 4cm.

With the default settings, it should hold about 30 10mm-high bobbins, since from `2 * π * r ≈ x * 10` with `r = 50` follows `x ≈ 30` (31.415, to be precise).

(More accurately: `2 * π * r = 31.415 * 10 ⇔ r = 50`. I planned to fit ~30 bobbins anyway, and liked that π gets canceled out instead of having a real number for diameter.)