Bev,
I've made a bunch of thread spool adapters too. Many of the threads my wife has bought has 1/2" or bigger holes in the spool. So I've used the 3/8" dowel rod to made fillers so the big hole threads don't wobble and tangle the thread. Then I've drilled holes in bottle caps to put the little long and narrow Sulky thread spools on. Those spools tend to let the thread drop off the bottom and wrap around the spool pin. The cap turned open side up prevents that.
Maybe I'll do a post on thread widgets.
Joe