Why the water bowl has to go
A slider needs humid air in transit, but loose water in a moving vehicle is a poor way to provide it. A bowl sloshes at the first bend in the road, soaking the tub floor, chilling the turtle as it evaporates, and leaving the air barely more humid than when you started. The answer is to carry the journey's water inside something that cannot spill — a material that holds moisture and releases it slowly into the air around the animal.
The damp-lining method
Line the travel tub with sphagnum moss, or with a towel or a thick layer of paper towels, soaked and then wrung out firmly. The test is simple: it should feel like a damp sponge, and squeezing it should produce no dripping water. Set the slider on top of the lining. Just before the lid goes on, mist the inside walls and the underside of the lid. The trapped moisture keeps the air humid and the turtle's skin and eyes comfortable for hours, and nothing can slosh however the driver brakes.
This semi-dry travel is not a compromise you are inventing under pressure — it is how keepers routinely move aquatic turtles between homes, vets and shows, and a healthy slider handles several hours of it without difficulty.
Balancing damp against stuffy
Humidity without airflow turns stale, so the lid needs genuine ventilation holes; the goal is moist air that still moves. Keep the tub level, shaded and out of direct sun — a closed tub in sunlight overheats dangerously fast, and that risk outranks humidity entirely. Position it somewhere it cannot slide or tip when the car turns.
Choosing the tub and lining
The tub itself should be just big enough for the slider to turn around in, with a lid that clips shut — a turtle travelling in a cavernous box slides and bangs at every turn, while one in a snug tub braces and rests. Lay the damp lining deep enough for the claws to grip, and pack any spare corners with extra towel so the slider cannot be thrown across the tub. If the journey crosses a cold day, wrap the whole tub in a blanket: damp lining holds humidity, but it holds no warmth of its own, and a chilled slider arrives stressed however moist the air was.
On longer journeys
Stop every few hours, open the tub somewhere safe, and re-mist the lining; if the moss has dried out to crispness, take it back to the wrung-out damp stage again. At the destination, offer a proper swim and a drink before food — rehydration first, dinner second. If a trip stretches across a full day or longer, plan an overnight stop where the slider can have a real soak in a basin of dechlorinated water rather than spending the whole journey merely damp.
Putting this wrung-out-moss method into your slider's Care Pass on KinsLe means that if a friend drives him or a boarder receives him, the instructions for keeping his tub humid travel with him word for word.