For seedlings that are still in flats or cell packs, I water them well, put them in a large rectangular plastic mesh laundry basket, and put the basket inside a plastic dry cleaning bag. I leave the bag open at a short side of the basket. This has kept my seedlings going for at least a week.
Boy, I wish I had known this trick a week ago. I took 3 flats of my sweet seedlings to Grandma's house four hours away in the car with 4 kids. They all did fine (the kids too) but there were some interesting times to say the least.
Thanks for the tip. I'll try this next spring break!!