I recently encountered issues with transporting large live bait. I had a baitsaver setup in my 5 gallon pail which recirculates the contained water via a spraybar. Works real good until the battery dies, so you have to carry a significant supply of small 12v batteries or a large one (weight factor)
While I have not built this, I found a pump for cheap on Ebay (item#110278330238) It is a 3gpm 12v submersible pump rated for fresh and salt water. It is tiny, and will fit in a toilet tissue tube with room to spare. I got it for $12.65 to put in my campers water tank to eliminate the hand pump on the sink. Thinking yesterday about my bait troubles, I wanted to build a better mouse trap. This pump is small enough to mount through a scupper hole. Mounted just deep enough, it could supply a new supply of water to the bait tank. The bait tank coule have a hole in its side to maintain the specified water level while fresh water is added. 3 gallons of water a minute is a lot, so you will probably want to cycle this operation as needed. I do not know how long these pumps will be available at this experimental price, and I have seen them listed for $45-70 online. I wanted to give the do it yourselfer a heds up should you be interested in this idea. I am going to look for an interval timer that runs on 12v that would make an automatic operation of the water change. I will post a heads up should I come up with something.
Good Luck
MAX