Here's mine. Not sure if this is what you mean by portable but mine just comes off the boat when not fishing. Gets water from the baitwell pump I had installed. Drains out the scupper. 20 Gal. I have kept threadfins, pinfish, greenbacks all day, no lossess.
Thanks for the responses. A 20 gal to 30 gal container that I can easily put on or take off the boat is what I have in mind. I have a blue barrel that I could use or cut down if I don't go the commercial tank route (John, I appreciate your generous offer).
Actually i'm using the garbage can he has in the background. We have it plumbed portable too - we have a cheap 500gph bilge pump connected to a 1/2 bilge pum[p hose. We either connect the bilge pump to a battery booster / jump starter pack ($20 at walmart) or we direct wire / clip it to the battery terminals.
The overflow hose goes over the side. The blue garbage can works PHENOMENAL for bunker - adults and peanuts.. We have had as many as 50 live adults in there and they stay frisky and lively all day long. Bunker are VERY VERY easily stressed out and killed, and blue tanks usually keep them pretty calm.
We've been fluke (ok flounder fishing for you peeps south of the mason dixon) fishing with live peanut bunker, and my 32gal barrel can easily keep 10 dozen or so alive all day long. We drain it and stow it in the Vberth when not in use.