I would just take a piece of string, twine or fishing line and fold the placenta up in half (to about the mares hocks) and then tie whatever you use around it.
The weight of the placenta on itself will help to stimulate the uterus to release the rest of it.
Also, the placenta should look like a big Y. So check it over to make sure there are no parts missing. If there is, i'd be on the phone with your vet and get them out right away before complications start. Those complications can come on VERY quickly...Within hours...
But really...99% of the time, there are no issues and a mare knows what she's doing.
Have you been taking her temperature morning and evening? Seen any reddish/brownish discharge around the vulva?
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