Though I don't have advice in terms of your DIY approach, I built an ice bath out of a giant garbage can and used an aquarium chiller to cool the water. The chiller can definitely get the water down to 40 degrees, but unless you pay a lot for a high-powered one, it takes several hours to get to the target temperature. Also, make sure your setup is connected to a GFCI power outlet. It might save your life if something…
Thank you for the advice, do you know the specs on the aquarium chiller by chance? I do plan to use a GFCI.
The one I bought is the Active Aqua AACH10HP, which is 1/10 horsepower. There are more powerful models, but they get pretty pricey.