Anyone else use the 'storage' areas either side of the boot? I have waterproofs in one and wellies in the other (just in case). I was playing with the idea of fitting a shower in one... A small water tank with a 12v pump and one of those travel showerheads should fit. Enough to wash the sea and sand off or clean the mud off your boots.
Gerbil, the 20 litre Really Useful Storage is perfect height for fitting between the boot floors. You can put a bunch of stuff in it to stop the stuff sliding about.