I used the spool holders successfully with 608 skate bearings and the stock 6mm threaded rod. Inserted a shim between the rod and the bearings! Also used a M6 nut encased in a nut knob (yellow) found elsewhere on this site.
Couple of plates
5 or 6mm threaded rod
Nuts for threaded rod
Plastic tubes (empty pens are great)
6mm glass/tile bit
Built with 626 bearing. Needed to add some internal dimensions to spacer to tight of a fit with 6mm threaded rod. Spacer had to be extended in Z - Axis only, to use with longer core. Directions confusing but end product works great.
Store these clamps apart from small SK® clamps as they appear the same with the exception of a threaded rod channel which allows attachment to a 6mm threaded rod.