Refrigerate it - don't freeze it.
You can use your hand or a teacup or one of your good silver spoons but I think a plastic bag may be a bit more acceptable to your parents - CoolioMom and CoolioDad.
If you want a good laugh, you could also put it into you sisters retainer case.....
I'm glad your cleared that part up about your sister and going to the vet. From the post I quoted it seemed as if you were taking her to the vet in lieu of the monitor.