Yeah, I missed the mark on that one. I tried making one myself, but it needs check valves to keep the water from going into your compressor or air into your water line.
I bought the $15.00 job, but forgot that it doesn't hook up to the water line.
If you just have a couple jobs to do, like power washing your house, you could rent the real thing for a few hours. Do your car engines too while you have it. Perhaps your neighbor would go halves on the rental.