Taking a potty training toddler to a festival?

My daughter is 23 months old, and doing great at potty training. She is in underwear all day, and hardly ever has an accident. We are planning on going to a local festival this coming weekend, and the only bathrooms available will be porta pottys. While I want my daughter to continue wearing underwear, I am VERY apprehensive about her using a porta potty (for the obvious reasons). Are there any parents out there who have any suggestions as to what to do? Is there anything you can buy to put on the portapotty seat so shes not exposed to god-knows-what on there while holding herself up? Thanks!