There's no reason why you couldn't build one, but without trial testing, there isn't a good way to tell what your heat transfer rate is going to be on a completely custom built unit.
Copper has the best heat exchange rate of any material that is readily available to the common person, but in order to calculate your exact heat exchange rate, you're going to need to know a lot of information. You need to know the flow rate of your water in your tank through this make shift heat exchanger. You need to know the temperature of your cooler water as well, along with the thickness of your copper tubing.
Simply put, you can just run your tank water down into a sump with copper piping, put cooler water in the sump, and by testing know what temperature you need to keep the sump. It may take some time, but I think you can figure out how to control the temperature during a day. Maybe you'll need to pour a new batch of water into the sump every day to make sure it cools enough.
At that point, is the work worth a custom built unit? It might be easier to buy one.