Using the square flower buckets, I opened another four Pound Shop packs of Coco Peat or Coir and cut the top of the bags ready to add the warm water to rehydrate them at home as it's faster using the electric kettle that holds up to 1.5 litres.
To give you some idea how the coir expands as its rehydrated
Top left box to be hydrated
Bottom left box has had its full 3 litres of water
Bottom right box hydrated before top right box but only with 1.5 litres of water
To assist in holding these buckets down in the boot of the car to prevent them falling over and spilling the coir, the bag was folded over and paving blocks placed on top.
The paving blocks need to be taken from home to the plot anyway so a win win on that one.