I noticed at least one response to the Porta Pottie survey saying their
Porta Pottie was unsecured when traveling.
Having been in a rolling (sideways, not forward on the wheels) I can tell
you that ANYTHING not secured will end up where you don't want it.  And if
you have any forward motion thrown in, expect any unsecured load to end up
in the front seat, right where you are.  So if there is anything in the back
you don't want dropped on your head GET IT TIED DOWN!!