I think reelescape1 has the right idea. The barrel should be cylindrical rather than oval. Take special note of the jets on his tank. They are jetting to the side creating a swirl at the perimeter of the tank. This makes for even flow and a constant current which allows bait to spread out in the tank and get equally oxygenated flow, and the don't beat each other up as much. Especially important when you're trying to keep big bunker alive in warmer water.
The 55 gal gals might be overkill for your application and another 440lbs at the stern is probably a heck of a lot more than you really need back there considering you've already got twins and a couple of guys fighting big fish God knows where. 30-35gal is probably plenty and ~ 200lbs less.
If you want to make your own, a good source of these barrels are car washes - they get soap delivered in them and they are tough! If you really don't want a thru-hull, a transom mounted bait pump is removable (uses a bracket) and can give you 700-800gph for about $80 from Rule or Kodiak. This tank is on the way out 'cause like I said I prefer round tanks, but this came with the boat, works and is portable. Note the brackets at the base - 2-4 short bolts and it's out of there. You can even release the pump from the transom and drop it in the tank to drain it... Good luck! Oh yeah, two drains is also a good idea - if scales, or grass, or belly-up bunker clogs or slows one down you're half as likely to over flow...
