K.N.
When my daughter transitioned to the real toilet, put her little plastic potty in my car trunk, lined the 'bowl' with a kitchen trash bag (kept a box of trash bags in the truck also and tissues ,hand wipes). If we were out and she had an "I gotta go" we'd go to the parking lot... Oh, I know... I'm the Queen of the parking lot potty! However, she was scared of public toilets because they seemed so loud and I didn't want that to interfere with potty training.
I myself would not want to carry around a toilet insert when I was at the mall, etc. Besides being bulky, it would gross me out, in that it would pick up any germs that are on the public toilet.