Black Water backing up in shower when driving.

Next time, don't turn it over and shake.

Sorry, devil made me say it.

If you are going that route, maybe a couple soft rubber balls just big enough to insert into the drain, with fender washers top and bottom and a bolt through it you can tighten to squash the ball so that it doesn't dislodge. Stick one in each drain, tighten the bolt a bit, and you have a sealing stopper.