When DD was younger, we used this fold-up cushioned potty seat and it worked really well. A friend of mine is using one with her newly trained DD now and it works well for them too. It comes with it's own bag so you don't have to worry about finding something to carry in.
http://www.walmart.com/ip/Mommy-s-H...fault&beacon_version=1.0.1&findingMethod=p13n
http://www.walmart.com/ip/Mommy-s-H...fault&beacon_version=1.0.1&findingMethod=p13n