You could just use a towel to cover the bottom also.
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You can use washable shelf liner if you want to give them some better (non eatable) footing.
They usually have it at Dollar General or Walmart
https://www.walmart.com/ip/EasyLine...MI1dTnqp3p4wIVEZ6fCh3QgQDHEAQYAiABEgJAQPD_BwE
It is inexpensive and cleanable.
You might get a get a big cardboard box or two that could be soiled and thrown away or put an old folded towel in the bottom of the tub.sadly, the container is plastic, so I worry that they will just slip everywhere.