Okay. I faced this dilemma previously when transporting a Serrasalmus Marginatus (spotted piranha). Some LFSs have like a lined box. It's cardboard on the outside, but lined and water proof on the inside. You fill it with tank water, and then seal it. You could try and get that. If not, I would recommend maybe a big cooler, or a large bucket with lid. Make sure the water is cycled, and that you can keep the temp up. In that case maybe a large rectangular cooler, like 55g would work. Keep in mind that piranhas will bite through plastic bags. Mine bites through them to get to feeders floating in the tank to climatize, and when I transported him he bit through the bag the guys at the LFS attempted to put him in. The last thing you need is a rhom thrashing around the inside of your car biting indiscriminately while you try and keep it between the lines. A full grown rhom could probably take a smaller hand off at the wrist, do be careful. Good luck.
