hey mate i just done this myself, watched a few youtube videos and blended the info into my custom. good choice for choosing a esky, its insulated already and wont heat up like those horrible alluminium tanks. i done a 1/2 " poly conduit spray bar with the holes all along it , a t-junction in the middle with a hose direct to the bilge which is the pick up within the esky, i fitted a two valve to control the flow but it isnt needed so essentially it is recycling the water and every now and then just grab a bucket of fresh seawater and add to the esky, the overflow isnt a problem if youve a self draining deck like mine, i found this easy to do rather than mount the bilge externally at the back of the boat. works a treat and this option is portable as the bilge is inside the esky and is wired with battery terminal clamps, plus the 20 amp fuse. however if you go the external option, i had it on my last boat and was great as well plus it had the deck wash hose branching off from the pick up too.