This spool holder mount fits cleanly on the 40x40 extrusion bottom rail of the Ender 3 (and any other printer that uses standard 40x40 aluminium extrusion).
The 30 degree version gives the spool more clearance to the printer frame but requires that the printer be right at the edge of the table as the spool hangs down beyond the table top. The 45 degree version gives plenty of clearance to the table top but is tighter to the printer side.
You will need two M5x12 mm bolts and two M5 nuts for installation in addition to the two M5 bolts and T-nuts that are already included with the printer.
The 30 degree version only leaves clearance for a 145 mm diameter spool (with 60 mm inner diameter) to the tabletop.
The 45 degree version provides clearance for a 206 mm diameter spool to the tabletop.