Best way (& I've moved half-way across country with a car full of snakes this way!) is for each snake to be in cloth "snake bag" (or pillowcase), one with no holes!
Use a twist-tie around the top, then fold that part over & secure w/ rubber band. (pillow cases are very hard to tie & un-tie...) No escapes ever this way.
THEN, put a little padding into a "ice chest" aka cooler/ or styrofoam box (like frozen food is shipped in), and put the snake-in-a-bag inside for transport. This is
for the safety & comfort of the snake, to minimize temperature extremes. Otherwise you may forget about the sun streaming into the car on the drive to & from,
while your snake cooks in the plastic box...or in winter, in & out of cold car does them no good either, health-wise. Please INSULATE them.
I know of some sad stories about snakes dying from improper transport (like the one that over-heated on the floor of a guy's truck) so please don't be one of them.
I've taken snakes to & from programs for years this way, and even kept them safe during an ice storm with power out for days (used jars of hot water that I replaced
every 8-9 hours to keep them warm enough).