We've always just done a bucket of water from the ocean and never had an issue cleaning them the next day, even if we are driving them 2 hours home and cleaning after unpacking the car. We keep the bucket 2/3 full w/ a lid, they travel fine. You do not need a bubbler for oxygen like crab, they're pretty stable.
Ice works as too instead of water, but is slightly more effort having to pack and plan for it. And if you use a bucket of ocean water, the clams filter out a lot of sand and are easier to clean in our experience.