I am not a mobile therapist but if I were, I would put my pot(s) and ancillary products in a sturdy plastic crate in the foot well of the passenger seat. If the crate is packed properly I can't see why the pot would get tipped over.
Roller systems are all very well except they are not actually that kind, they can drag the skin and they are also not as workable as a spatula in undulating areas. (Think large client, bikini line!)
They are probably also less profitable and lets face it, we work to make a living!
Just one thing I would like to add for SparkleJeanie, you may want to take something to protect the clients floor. Either a towel or maybe a shower curtain.