İf you can find a cheap, used small fridge (like the ones used in dorms) you can build your own easily.
take out the freezer compartment if there is any, put about 25 meters of thin walled hose inside the fridge, drill two holes on the top for inlet and outlet and take out the two ends of the hose through them. You can use a small aquarium
pump to circulate the
nutrient solution through the tubing inside the fridge. Don't forget to seal any gaps around the holes for efficient cooling.
This should be a pretty efficient cooler and it costs practically nothing besides the fridge. There are used dorm fridges on ebay under $50. If the thermostat is intact you can control the solution temperature automatically.