How do I keep my spools of thread from wobbling off the spool holder and spool pin of my serger?

I have a Janome 204D (sim to the Janome Magnolia). I use Maxi-Lock thread. The spools of thread are running low, therefore lightweight, and are starting to wobble severely even though I used the spool holders that came with the serger. I guess the brand Maxi-Lock has a larger bottom opening than anticipated by Janome's spool holder. Any suggestions?? I hate to switch brands because of the price and I dont see any larger spool holders -- I am in the process of finding the spool caps; thinking they might help the situation. Thanks in advance!
"SPOOL HOLDER" IS A 'PLASTIC INSERT', PLEASE READ THE QUESTION!!!!!!!!!!!! THE PLASTIC INSERT/SPOOL HOLDER FITS ON THE SPOOL PIN, THEN YOU PLACE THE THREAD CONE OVER THE SPOOL HOLDER/PLASTIC INSERT. MY SPOOL HOLDERS/PLASTIC INSERTS ARE TOO SMALL. NOTHING ELSE CAME WITH MY MACHINE AND I DONT SEE ANYTHING ELSE AVAILABLE

The spool caps should help, but there are also plastic inserts that you can put inside the cones that will make them fit better. I'm pretty sure that you can get them at most sewing or hobby stores.

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