You should look for a barrel in the 30-gallon range like this :
If you are resourceful, you can find one for free. We have the pickleworks factory in Staten Island, who are willing to let them go for nothing. My friend in Jersey gave me one he got from a cleaning company - it had soap in it.
Take the washdown hose on your boat and use it to pump water into the barrel. You will have to cut a hole in the top, and rig up a standing drain using pvc pipe. It has to drain from the top so that enough water stays in it.
I'll be building mine in the next few weeks. I'll post photos as I go. This should easily keep about 50 bunker alive (you could probably double that number). If you are going to liveline, you may as well do it right.
My barrel will actually fit between the leaning post and the console on my 20-foot center console.