I used to be into big reef tanks, and investigated chillers from that perspective.
DIY chillers generally don't work. If you use plastic/rubber hose (for example running it through a mini-fridge) you don't get good enough heat exchange. I'd beware of using chillers made for a drinking water dispensor, as it probably has copper tubing in the heat exchanger, and this will likely leach copper into the water over time, killing your octopus.
Real chillers made for salt water use titanium heat exchangers, which is part of why they are so expensive. But you need to use something that won't corrode in salt water (or leach poisonous metals) and passes heat fairly well.
The best solution in many cases is to put a window air-conditioner in the room that houses your tank. That way you don't need something special for aquariums, and you can enjoy the cooling too.
Another solution that I've used a couple of times is evaporative cooling (i.e. swamp-coolers). The idea is to put one or more small fans so that they blow across the surface of the water. If you do this right, and the humidity isn't too high, it will evaporate over a gallon of water a day from the tank, but cool the tank at the same time. Of course, if you have an octopus that likes to try to escape, you have a problem, as you won't get good air flow with a tight fitting cover. I've used computer muffin-fans for this when I've done it. Be warned--they are called swamp coolers because it will make the rest of the room warmer and more humid (swamp-like).