I printed the large Version of this Spoolholder.
In order to mount the Bearings I used a threaded rod and cut a thread.
In one end of the rod you simply make a slot for a screwdriver.
I used some pliers first to make the thread and then cut the rod to length.
Since I have a few Spools that are not smooth on the inside I made a pipe that fits over the Spoolholder.
If you choose to go that rout you may be able to use just 2 ore 3 bearings.
This is a really nice design and works like a charm.