A refrigerator, a Styrofoam or Styrofoam lined minnow bucket and a tropical fish tank air pump and bubble stone works quite well for me keeping croppie minnows alive at home.
I'm fortunate enough to get to fish a couple times a week and have very few minnows die between trips...I have kept them alive this way for a week or two and they seem to last quite well during the fishing day also. The frozen water bottles sounds like a great idea for those long fishing days.