These spool bushings are simple. Use one or two elastic bands to install them on a spool. Nothing else is required. The center hole of the bushing is for a 1/4" smooth rod. It can be easily increased with a drill bit.
I have supplied spool bushing of 31, 39, 52 and 54 mm. They fit center holes of 32, 40, 53 and 55 mm respectively. If you need another size, just give me the diameter of the center hole of your spool, and I will upload them. The minimum size is 30 mm.