I use a small dog carrier and my two piggies always huddle in there, too afraid to move. At one point, I had three piggies and had to use two carriers. It was a little much taking two carriers on public transit.
I've tried multiple carriers, including a home made version. The worse was a backpack one. Whenever I forget and bent over, the poor piggies would go sliding away. I'm trying to give that carrier to my sister for her dog.