When moving large fish i advise adding a few oxygenating tablets to each bag before you seal them up, you should be able to buy the tablets from the lfs for a couple of quid or so. I wouldnt advise using binliners to carry the fish in case the plastic is not fish safe, if possible find a fish store that specialises in koi and ask to buy some of their jumbo bags from them, they normally charge about 12 pence per bag. If a bag does not fill the box completely stabilise it by adding bags inflated with air either side of it to keep them upright, also when filling the bags with water remember 1/3 water and 2/3 air to allow the maximum surface area possible within the bag.