Since you did not tell us how long/how big that ponytail is, I am taking a guess.
If it is long enough, you could try loading it up with some sort of styling product like Ash suggested, braiding it, and coiling it, and pinning it into a small bun that the Saber hairbun will fit over.
Another option might be to get a hairnet and securing that over the ponytail so the fibers are contained, then putting the Saber hairbun over it.
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