Black Water backing up in shower when driving.

Peg Leg wrote:
Flat rubber stopper over drain with a weight on top?

I would go the route of pulling out the plastic water stops that don't really seal well during a "back-up" situation and use black rubber "corks" that are inserted into the top of the drain and forced in. Should hold until you can figure out why it is backing up. I believe the big box hardware stores should have those stoppers in the plumbing section.