Some of us have added a shower to make the end of the day that much nicer. It works great but is a little big and heavy. I will try a smaller one next. Try to keep the direction of flow opposite each other for max heat transfer.
Position fittings so that no air is trapped.
It doesn't seem to make any differance if the engine coolant is on the outside of the flex tube and the water inside or the other way around. We have been using a garden hose and for the suction side and air hose for the output side with a sink sprayer or shower head with on-off valve. Use a filter on the suction hose. We are using a cast metal sprinkler with very small holes in it's face, for a filter. An on-off valve is handy when water is in short supply.