|Hi, This is my first post! :)
I bought a janome serger and wanted to use regular sized spools/thread. I didnt have any spool caps and I couldnt find any at my nearby sewing stores, so I improvised and used 4 smooth 1/2" by 2" washers (donut shaped things!) from lowes. Worked beautifully!
Hope this helps somebody looking for quick and cheap spool caps!