The book that came with my kit states pretty straight forward that after the boil is done to put the lid on and move the pot to a sink with ice water in it. That was my approach. Will a sink of ice water, I got the wort down to 83F in 20 minutes. But it was a partial boil not full 5 gallons. The more you have the longer it takes to cool.
Making our own immersion chiller sounds like a fun project.