This filament spool holder uses 40 – 5/16in (7.9375mm) ball bearings. The ninja version adds a 1.75mm track along the bottom where you can insert 1.75mm NinjaFlex filament to keep it from sliding around (see the picture of the black version above). Should work with a variety of spool sizes, narrow spools (<55mm wide) can be placed across the longer length, wider spools should be placed across the shorter width. If 5/16in bearings aren’t available, you can probably scale the print slightly and use 8mm bearings. You should probably watch a while to see if it works with your spool before leaving it unmonitored.