Malek I do a lot of fresh water fishing, That cooler would work but it must be a pita to carry around. You said you are using sunnies and other small fish. I would suggest you add suckers to that list. They can be caught by enlarging the hole in a minnow trap and leaving near the mouth of small freshwater creeks that feed into the bigger ones. Another trick is to cut their dorsal fin on the bottom so they can't just sit in a hole and have to move around. It also slows them down for the catfish and I have found the catfish like softer baits like suckers more than they do sunnies though sunnies will work too.

I like that oxygenator never used it before but looks like some nice technology. If you use it be sure to give us a report on it. Like the 12 volt battery too. I agee the d batteries are crap for keeping bait alive all day when fishing in a boat I bring an extra deep cell 12v and use that to keep the bait alive.
Good luck let us know how you do. Whats your biggest flathead so far?