G asked me about this so I decided to post up some resources for him and all of you. There are several ways to go about it depending on what your needs are and how you will use it.
Budget bait coolers:
This is for the shore based fishermen who don't have the money to buy a custom bait cooler, or the guys who want to make a small portable cooler.
5 gal bucket
The simplest portable ones are 5 gal buckets with battery aerators. That's what I use when I'm creepin around the concrete and have to be mobile. Sometimes the 5 gals is way too small. For that reason I have an 8 gal, and usually carry 2 buckets with me. Nothing is more googanish than going to the trouble to get live bait and then having it die on you.
That being said, here are the ways I usually go about solving a problem like this:
1. What are my bait objectives?
2. What is the primary bait I will be dealing with?
3. Where will I be fishing?
4. Do I have a source of constant salt water handy to change out the ammonia and waste, or will that be difficult?
5. How much bait do I need at one time (that's why I sometimes have 2 bait buckets, keeping the bulk of the bait hidden somewhere accessible in deeper water.)
6. How involved does this need to be?
Simple is best, and different answers to the above will cause you to design or set up different systems.