I've kept them alive for a week in a big bait bucket...you can feed them shrimp. Very hardy...just make sure they get plenty of oxygen. Free flowing water is idea...I would reccomend cutting holes in the bucket and adding a top and keep tied to a dock.
I've kept two dozen or so alive overnight in one of those big pool chlorine buckets with a portable aerator, but this was in the late fall when it was cooler. I would definitely make sure the bucket stays out of the sun and maybe even toss in a frozen water bottle to keep the water from getting too hot.
I've kept them for a week or more in the garage in a big plastic storage tub with an aerator and a small cheap-o aquarium filter. Filled it up with the water hose, put some chlorine killer in, and dumped in a bunch of sea salt (yes, the cooking kind.) Not the most scientific setup, but it worked like a champ! Also keeps shrimp, mullet, mudminnows, etc alive when you have leftovers between trips. Just keep 'em separated.