It doesn’t sound any different than taking a regular bath but it helps more because you’re elevated in the water. Helps with healing and just feels soothing after a bad flare up. You can find them on amazon or at your local pharmacy - I have this one! I got the same one at my pharmacy for about $14.
I don't know if there is something specifically made for the car, but I wonder if a sitz bath tub would work if you did some adjustments? I feel you on those DC traffic issues, it took me four hours 0.0 to get home Wednesday.